Monsters have been appearing on already uncovered tiles as I try and move onto them, instead of being visible beforehand like they usually are. 

This looks like one of the monster icons failed to load when you entered the island. Usually this happens when your internet connection is unstable. If this is not the case, we need more data from you: a link/ID of the exploring pet, the time when the explore session happened, and which monster and where it was.

Ah that makes sense. It's working as usual now, thanks for your help!