ZDoom

Thanks. I had no idea that there was more than one Nerve.wad floating around.

This is not the original version of nerve.wad, this is a modified version that was made to play it in PrBoom+ before UMAPINFO was a thing so as to get the secret exit on the right spot.

If you load the original, unmodified version of nerve.wad, the one where it starts on MAP01, then GZDoom will do some behind-the-scene trickery to allow the NRFTL maps to coexist with the original Doom II map (basically renaming the map slots so that instead of MAP01 it's LEVEL01 and so on). It only does that with the pristine nerve.wad version; however. The general assumption is that if you load something that replaces MAP01, you want it to replace MAP01, and if you load something that replaces MAP12, you want it to replace MAP12.

Well, look at the list and see where the NERVE levels are placed. The version you load overrides MAP12 and on - you cannot use this version and still play regular Doom 2 with it being loaded.

See attached log file. Hopefully that helps.

Also Doom II is loaded with the following files/parameters: "-file DoomIImetalost.wad Liquidpatchv2.pk3 AltDHTPv1.3.pk3 AltBrightmapsDoom2.pk3 anidoom_1.3.pk3 master.wad nerve.wad"

When "nerve.wad" is removed the problem is resolved.

You need to give more info what you load and how you load it and what maps you have defined afterward.
For that, please post a startup log containing the contents of the 'listmaps' console command.

Hopefully this hasn't been reported before. I couldn't find anything. Using GZDoom 4.9.0. Bug occurs when loading Doom II with the expansion episode No Rest for the Living. I like to load both No Rest for the Living as well as Master Levels when loading Doom II. Error occurs when completing Map 11 Circle of Death. If the expansion pack has been loaded (but you are playing Hell on Earth) you will not proceed to Map 12 The Factory. Instead you will proceed to Map 1 Earth Base which is the first map from No Rest for the Living.