Looks likes, the engine have some problems with rendering slopes through mirrors. Also, there is a weird bug with changing things height while player crouches – it appears even without mirror.
Additionally checked it on 1.35 and 2.0.4 (standard and SSE-2) - problem looks completely same.
As usual, demonstration video: https://youtu.be/SpRjcMCRiw0
And test map from video: https://1drv.ms/u/s!Akj1GftRhTs2grdUZaIerlqmAnCWWQ
All right, I pushed a couple fixes related to this report. The wall tiles in the mirror are no longer swapped. The coord transform deals with that making the extra swap flip things. I also commented out the code forcing an update of the position of items when the player crouches or stands. The update doesn't understand slopes, so it was forcing the items to go to where it thought the sector floor was supposed to be. I'm not even sure why that code was even there considering the next render of the frame totally deals with the position and height and angle of everything in the player's view based on the current height of the player.
Anyway, that's two-thirds of the bug report. I'm still looking at why the player clips at certain offsets in the mirror.