Why does this happen?

[Sussudio]

Since I don't know if this is to be expected because of a limitation in the engine, I have refrained from posting it in "Bugs"

I summoned a cyberdemon (but happens with every monster expect for the floating ones) on a 3d-floor elevator, but it would lose the ability to see me when the elevator raised. Sometimes it would see me for a few seconds as I ran around the place evading the first projectiles, but then it would stop the aggressions all together and basically become blind as in the first example.

Can't remember who created this wad but kudos be to the author. Check the save file (r604) for the already blinded cyberdemon.

Thanks.