I've seen this happen several times now, particularly in the big room, if you are surrounded by a horde of creatures, they always stay on the right side of you, bunching up there.

For example, in big room, I'm right now standing against the top of the map with a horde of jackals. It doesn't matter which jackals I kill or if I just wait a turn, none of them will ever move towards the left to surround me. If another creature wanders in from the left of the horde it will come up next to me, but, these jackals will never move from the right side to the left side.

Code:

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..@ddddd
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I can also move down like this and they won't move into the space above me or towards my left.

Code:

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...ddddd
..@ddddd
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If I move one more down, then they will move into the square above me.

Code:

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...ddddd
..dddddd
..@ddddd
..dddddd