I was playing r57. I downloaded r59 from the public site, and did not see the Challenge Mode option.
Just downloaded r58 from the pre-release site, set the date to June 5, and made it happen.
Specifically, once I saw the stairs, I walked directly there. Then moved away once (took damage), used first aid, moved away again (took damage), used first aid, moved back again.
Expected result:
no damage
Result:
took damage.
I moved directly away, no diagonals. Just to the right twice, then directly back once to the left.
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the save file is from r58, a trollish assassin, just after the stair damage was applied incorrectly.