Sometimes the tour guide freezes after reloading the game, and I encounter the same problem. I always fix it temporarily by putting the guide in the break room. Then he takes a break and then continues as normal.
I have also encountered this bug a few times and I think its been across multiple museums, so not restricted to one museum/theme. Same issues as above, the expert giving tour guides does not take breaks, so then you get the notification they are close to resigning - need food/drink/toilet.
I always tweak my experts to just do tours if they have the eloquent trait, and ones that don't do them, but I have multiple so its not a case of an expert not being available. They just seem stuck in a death spiral/loop!
This has happened to me several times. It looks like the expert guiding the tour gets stuck along the way and just stands around until their needs are depleted and the game warns you about bad mood. I've seen experts stand around for months at the same spot without moving the tour forwards. Reloading the map seems to get them unstuck, but shouldn't be necessary.
I’m not sure if this is a bug but my tour guides are quitting. Their happiness is getting low and they aren’t taking themselves for breaks. Ive tried removing all other tasks, keeping the tour short, sending them for breaks. I have toilets and food for them but it doesn’t help and I have to micro mange to keep them in the museums.