ZDoom

Many original levels would not work without that flag. Been there, done that, mess ensued.

I don't quite understand why is there limited behavior in a newschool port without a compatflag, neither this limitation makes sense (at least for me), but okay, my bad, didn't know about that. Sorries.
Besides, are there iwads or mods that actually rely on this one-sided z blocking?

You should do some reading on the MF2_PASSMOBJ (CANPASS in DECORATE) flag. Without this, z-aware actor checks are intentionally limited.

Tested on zdoom-2.9pre-1000-g9cf1d96, but seems to happen since ancient times (at least ZDoom 2.5.0, probably earlier).

The way I found it, it's impossible to have a player and some other actor to occupy the same X/Y coordinates, no matter what, with infinitely tall actors [b]DISABLED[/b].
Even weirder, the player is able to move above the other actor, but the other actor can't move above/below the player.

Minimal example wad here:
http://www.mediafire.com/download/ticfxo3dy6uffvg/BUGGYNAP.wad
SetActorPosition() will fail on the barrel, and if you use "fly" cheat and try to hover above the barrel, it will stop moving on the scrolling floor (but you can still pass above it and it will continue it's movement).

Note that if I change the actor to something that is supposed to fly (i.e. a Cacodemon), it will be successfully teleported to above the player AND THE PLAYER WILL BE ABLE TO MOVE ANYWAY.