Creating an E1M3-esque pool elevator in UDMF?

Howdy. I'm using UDMF, and I'm trying to create one of those cool rising floors that starts off as a damaging acid pool, but rises up to the nearest floor and becomes a harmless... well, floor. But for some reason, it rises up and the damage effect stays. Anyone have a clue what's going on? Do I need to do something more complicated that I would've in classic DOOM mode?

The action used in E1M3 (linedef type 20) corresponds in Hexen format or UDMF ZDoom namespace to Plat_RaiseAndStayTx0 (tag, 4).

Thanks, Gez! You're a saint!