I almost bet the GM found a map they thought looked cool and is just throwing stuff at you to fight because it’s faster and easier than coming up with storylines and characters with motivations. I get it, we GMs get busy with life and stuff too, but if you’ve been doing this for a while, there’s always a reserve of characters or stories that you made but didn’t use which can be pulled into the current game. With a new GM, they may not have that luxury.
All that said, the best course of action seems to be the thing you’ve already concluded: talk to the GM and the group. Maybe let someone else try being GM if it’s just the case of the current one being busy. As others have said, running a prewritten campaign is just fine, especially in Pathfinder. Heck, you can even buy many of them as foundry modules with music, maps and handouts all ready.
I always over plan things. I’ll plan encounters appropriate to level. I’ll plan NPCs. I’ll plan dungeon themes. But I won’t plan a dungeon themed encounter unless I know they’re heading into that dungeon because it’s where we ended the last session.
To put it another way: I never plan so specifically that a thing can’t be moved to another place unless I’m positive it’s coming next.