Stepping on an altar will make it disappear graphically
Included is a save, showing it happening with a lawful altar (or neutral, if you sac the demon). If you step on the altar, then step away it will not show up on the ground. The altar doesn't actually go anywhere, it just isn't displayed properly until you redraw the screen (either with Ctrl+r, or by leaving and returning to the level).
I noticed this in the OSX version too (and even in the Amiga one, even though this one uses a different cursor approach). Guess I have to rethink the way the cursor is handled.