I'd also consider this a bug; as I intuitively interpret Slayer movement, Slayers don't "
take into account"
the fact that tiles are inaccessible as far as what defensive actions they take and what they think the player is capable of doing; they merely check what moves are open/blocked when they feel they have to make a defensive rotation/move.
Thus, I don't see any reason why they should be able to distinguish between the room boundary and a regular wall / force arrow / monster-blocked tile / etc., no other monster does that. (Except puffs but they're special.)
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