Monsters do nothing bug
My first theory was, it had something to do with the slide mechanic and that it somehow bugged. Performing a simple slide should of returned the player to his normal height and monsters would see the player again, since that was the only thing I could imagine what went wrong. However I did counter what I believed to be the problem in later versions, with a script that always resets the player height on entering any level.
Unfortunately that didn't seem to have fixed the problem. Since 1 persons had the same problem with the later versions with the player reset script included.
1. Method to fix:
A guest in the comment section said he was able to resolve the problem by deleting his GZDoom-User.ini. Make a backup of your user.ini just in case it doesn't work.
2. Method to fix:
A user on my discord wasn't able to fix the problem with a new ini. His method was to open the console and type notarger to activate it and type notarget again to disable it.
Actor cannot be targeted by other monsters.
Looking trough the scripts I couldn't find anything that would activate that but apparently that helped. I hope this helps. So far there have been 3 people reporting this problem and 2 were able to fix it. 1 has not responded yet. Feel free to comment if you have any knowledge on what might be causing this problem or what you did to fix it. Since I and apparently around 500 other people do not have this problem.