I've been playing just as long and have never seen that though I've seen some odd ones crop up which are very rare. Mostly in the Catacombs.

1) When returning to a previously played Catacombs level, to make use of a shrine, walls and doors are offset and the map which was completely drawn in when the level was finished now has gaps. Usually not a problem getting around.

2) When returning to a previously played level coming upon a Town Portal. I've seen this in both Catacombs and Cathedral. It''s never been clear to me if the Town Portal is one I had cast or one that appeared as a result of one being cast elsewhere on that or a later level.

3) When playing a level of the Catacombs discovering that a tunnel which should connect ends, walled off from where it should connect, and getting to the the rest of the level requires either teleporting or repeatedly phasing.

It occurs to me just now that the problem might be that the stairs aren't rendered but if one could find where it says "To Level X" one could proceed.

Multiplayer or single player?

I've never run into this but I've never played vanilla Hellfire. Let me know if you are interested in playing a more updated unofficial version of Hellfire I'm trying to find other people to play hellfire multiplayer with. I use a mod that updated a lot of things with hellfire, fixed a bunch of bugs and added a bunch of QoL stuff.

More info here:
https://www.gog.com/forum/diablo/adventurers_wanted_for_diablo_hellfire_multiplayer/post1