How do I prevent pathfinding enemies from getting stuck like this. Or how can I make them unstuck?

Every time they overlap a solid like this they're unable to move. I guess being inside a solid breaks the pathfinding ability..

How did they get there in the first place?

That would be where I would start.

You can also have an event to check if they overlap a solid and move them, but I wouldn't go to that solution first, as you might run into it later at undesired times.