ZDoom

My work around for this was to create a dumy actor, when activated it sets a flag to execute it's special on next tick. So I set the bump line to activate the actor, then the actor special to do the teleport.

Then I realized I don't want to bump a wall to trigger a teleport because it kills player momentum, so if I'm trying to simulate moving through the wall, there's a jerk in the motion. Rather than a solid wall, I set it to a non-blocking mid texture and use a walk-over line and silent teleport to keep the momentum fluid

Seems similar to an issue I saw when trying to create an actor that could teleport the player away when bumped, in this thread. The solution was to move the special call out of the collision detection, either flag it to call it later or spawn a thinker to handle it.

I suppose the solution here is to fake the bump, put a walk-over line at just the right place so it feels like a bump.

It indeed is and it is generally not fixable.

reproduce:
Create Sector A, floor 64, player start
Create sector B, floor 0, teleport landing
pick a line in sector A, set it to use and bump, special teleport pointing to landing (tested with 70 and 71)

Use the line, teleport as expected
Bump the line, teleport to landing (x,y) but z=64

Teseted with special 70 and 71
Tested with thing 14 and 9044

I assume it's some issue with the teleport happening in the middle of the movement code