This is a Zdoom bug, now reported here:
http://forum.zdoom.org/viewtopic.php?f=2&t=17856
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Noticed in map str03 of the Stronghold demo beta available from the Zdoom thread in projects.
This looks like the gl_clamping_bug problem, but toggling the cvar makes no difference.
I don't know what it is related to because I've looked at other WADs that use warping floors and they do not seem to show the problem.
Correct appearance in GZdoom release version

Incorrect in r127
