So here's a full impact report on what would happen if this were changed. My goal here is not to make a big fuss about changing it, only to note that if it happened, few rooms would be affected.
In a lot of the rooms, it's obvious with a quick glance that there's no possible situation where the firetrap is between the gentryii and the player at all. I'll start with a list of those rooms.
Another big set of rooms is in Octodecaphobia in the Hold Title Mixup Contest Compilation. These rooms all have the same pattern so I can cover them all at once.
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In each room, a two-tile gentryii is chained to a wall. Its sole purpose is to force you to maintain stealth (the level makes you a guard player role). If you break stealth, the gentryii blocks you from proceeding to the exit passageway. On top of that, you break stealth by killing the brain. So there's no situation in a winning demo where the gentryii pathfinds over a firetrap.
See the rooms for yourself:
Hold Title Mixup Contest Compilation - Octodecaphobia -
1 South
Hold Title Mixup Contest Compilation - Octodecaphobia -
Entrance
Hold Title Mixup Contest Compilation - Octodecaphobia -
2 South
Hold Title Mixup Contest Compilation - Octodecaphobia -
3 South
Hold Title Mixup Contest Compilation - Octodecaphobia -
4 South
Hold Title Mixup Contest Compilation - Octodecaphobia -
1 North
These two situations already account for 14 of the 29 rooms. The rest have a little more nuance. I'll cover them individually.
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Combined hazards - Gentryii & pressure plates -
3 South, 3 West
This takes place in a portion of the level where you're disarmed. In order to get to the brains, the four pressure plates with orthosquares have to be stepped on. The intended solution involves getting the gentryii to move over them - other options are probably impossible. Even if not, without a sword you can't get to where the brains are, and so you can't cross the firetraps under them. And even if there's some break in the scripting that allows you to bring a sword here, it's still clearly possible to solve this without the behavior coming into play.
Combined hazards - Gentryii & pressure plates -
2 South
Combined hazards - Gentryii & pressure plates -
3 South
These are variants of the same room. The brain in the middle is on the firetrap. Once all the gentryii are released, the roach will follow the brain pathmap and step on its pressure plate. This opens the door around the brain and turns on the firetrap. However, the roach moves after all the gentryii, so the processing order is gentryii move - roach opens door - brain dies on firetrap. This means there is no point where the brain pathmap will even consider the traversability of the firetrap.
Dugonium - Prologue -
Entrance
The player starts this room disarmed - it is the first room of the hold and the entrance is on the left side. The player is promptly pursued by gentryii. In order to complete the room, they must close the door by hitting one of the orbs near it. This traps a gentryii behind them. The firetraps (which kill the brains) can only be activated after this point. Furthermore, the firetraps are in corners so they are not likely to impact the pathmap even of the gentryii head that makes it past the door.
Gardeviance - Postmaster General -
1 South, 2 West
The gentryii are clearly separated from the firetraps, but it might theoretically be possible to do some island-hopping shenanigans with the platform to get one over to where the firetraps are. Even then, the most you could do is get on the other side of a firetrap on a totally linear path and I can't see how you could solve the room afterwards.
But if somebody somehow managed to pull that off, their demo might break. This is totally unnecessary to solve the room, however.
Gentryiification - Grisly Gentryii Gimmicks -
4 South, 1 East
A thorough analysis is hard to do here. But my demo never opens the doors while the firetraps are on, so the room would definitely still be solvable with changed behavior.
Pass the Hold Around - Contribution -
1 North, 1 East
The gentryii is chained up, but if you turn on the firetraps under the goblins, it's possible that could change the pathmap and make it move slightly differently, breaking demos.
However, the room has a secret access which requires not activating any of the firetraps in the room, so it would still be solvable.
Take Five - A Five Brains Hold -
1 South
The only purpose of the gentryii in this room is to oscillate back and forth. This repeatedly toggles the firetraps and changes the pathmaps of the constructs. This would not change if the gentryii had valid paths through the firetraps.
Take Five - A Five Brains Hold -
1 West
The only firetrap in the room is the one west of the gel baby. It is activated by the on-off plate the gel baby is on, which is the only tile past it.
In short, for the pathmap to be relevant you would have to somehow bypass the firetrap and kill the gel baby without getting killed by the firetrap yourself. Which is possible with player role fegundo, but this room is done with Beethro so it's not going to come up.
Temporal Adventure - Overgrown Ruins -
2 South, 2 West
The firetraps are under the briar roots. Activating them requires releasing the briar, which will cause it to grow over the doors and thereby block the firetraps.
Unchained Malady - Gentryii Head-ache -
Entrance
All of the firetraps are under kegs and surrounded by ice. Consequently, activating any firetrap in this room renders it inaccessible and therefore irrelevant to the brain pathmap.
Unchained Malady - Infiltration of Esterport Fort -
1 South, 1 East
Solving this room requires going past the two firetraps in the south passage, allowing you to step on a plate and activate the firetrap under the brain. I don't know for certain if there's some way to turn them back on after you do and still solve the room, but it can definitely be solved without either of them turning back on.
Unchained Malady - Mechanical Roaches -
1 East
The firetraps are activated when the player steps on the plate with a speed potion on it, closing the adjacent doors and trapping the player. This also causes the briar to grow, and it deactivates the firetraps two turns later. (Eventually it grows over the right plate and lets the player move again, but that's not really relevant to the interaction.)
On the first of those turns the gentryii has only one path to take. It might be that with a particular fluff setup the gentryii would move northwest instead of directly west, but in my demo it moves west after the firetraps are off so it would probably move west while they were on.
Even if the direction changed, it would probably end up in the same place by the time the player could move again, and there are no pressure plates it could activate in the meantime. At worst it would break some demos, but not render the room unsolvable.
That covers all the rooms aside from the two I noted in my previous post which actually depend on the current behavior. If the final verdict is that we're not changing it, well, I found it interesting to evaluate the impact anyways so I'm not going to be too bothered if it ends up not leading to anything.