Tar can exist on arrows. Curiously, this seems to occur only twice in the main holds (though I may have missed some), in two rooms of TRA. So I would not expect turning back on an arrow under tar to do anything (and indeed, it doesn't).
This behaviour has been noticed before, in bomber50's disabled arrows tutorial in Architecture, though he evidently didn't consider it a bug. I agree that it's odd -- in every other respect, a disabled arrow is the same as normal floor -- and it's one of those
exceptional cases I talked about that's annoying because it increases the number of rules you have to remember. So I would prefer it to be changed, for what that's worth.
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50th Skywatcher