Some oddity with original Doom's exit linespecials and if they have tag numbers assigned to them.
If the triggered exit line for some reason has a tag-number other than 0, the next level after the intermission screen starts from the player being at cordinates x=0, y=0, yaw=0, pitch=0. This may be outside the map area.
Here's a test map for Doom2 map01. Hit the exit switch or walk into exit. And after the intermission screen check out the location where you start map02.