I tried what you suggested with a slight mod:
I made 4 consecutive 1024 x 1024 boxes in a row with light levels 240, 255, 256, 240, I made a white flat and a white texture, I then rotated everything 45 degrees to prevent the fake contrast effect.
I checked the map in vanilla, chocolate doom 1.7, prboom-plus 2514 and zdoom 260. It all looks the same to me, I can't see any difference in light levels from any viewpoint.
Veinen's demo shows the same problem on MAP13 as has been mentioned for MAP12.
Here is a scan of the area around sector 777 in MAP13 (generated using R_PointInSubsector): http://i.imgur.com/HtjWuh1.png
The red crosses within sector 777 are sector 331.
Monsters will disappear through the floor if they stand on these points.
It looks to me as though the nodebuilder you used for maps 12 and 13 has created this problem.
This wad contains a DBIGFONT lump stripped of kerning information so that the
main menu items would at least display, although not properly positioned, in
ZDaemon. It also containes a fixed MAPINFO lump that gets properly parsed in
ZDaemon; the main effect this has is that proper wider sky textures are used.
USAGE: This wad needs to be loaded as another pwad after the btsx wad for it to have any effect.
NOTE: The issue with kerning has been fixed for the next release of ZDaemon.
Not sure if it's been mentioned before since I just got up but a buddy of mine found a few problems. He's going through with PrBoom+ in pure error mode to try catching more but for now what he found is a missed backside on get out of my stations 2 (map16); linedef 82 has two-sided flag set but no second sidedef. He says it's a non-fatal error but it causes demo desyncs.
Will report more as they're found.
Edit: MOAR STUFF
Gonna Never Have To Die: MISSEDBACKSIDE, linedef 1051 has two-sided flag set, but no second sidedef. Desyncs if you try playing a demo recorded there.
spechit overflow in map22, at the megahealth switch in the water. list of lineid leading to overrun: 1363, 1364, 1365, 1366, 4903, 4918, 1359, 1360, 1361.
Potentially fatal in Vanilla. Don't know for sure so someone please test.
Hall of mirrors on the floor at the lift of map24: tough skin river.
Possibly missing texture just before that.
No Name Map (Possibly the ending?)
Missed backside on the ending, linedef 337 has two-sided flag set, but no second sidedef