A little vertex moving will fix this.

This is a nodes issue, and theoretically it can be fixed if the proper node-building settings for ZDBSP can be found (though at this point I'm not convinced there's not an error somewhere in external ZDBSP considering how all options thus far have failed to produce perfect nodes). Using ZDoom's internal nodebuilder ("gennodes 1") fixes this, and while I guess you can jiggle the vertices around until this is done, there are several more spots around the map (including some that cause visual bleeding through walls and even clipping errors) that are also related, meaning a better nodebuild is the desirable solution so we don't have to stop-gap everything.

tl;dr: I'd suggest closing this and opening a general "Nodes are borked" bugthread, to officially put it on-record.

Something I wonder: if you use the dumpmap command, does ZDoom dump the nodes with it? If so, gennodes 1 + dumpmap == perfect ZDBSP.

Unfortunately not. In fact, it presents another oddity in that it saves the result in Hexen format, despite being UDMF originally. Since the wiki describes it as saving the "current state" of the map, I'm not sure it'd work well anyhow without botching something up.

We need a "dumpnodes". <_<

Moved the vertice, but this is more an enginerelated thing.