After advancing to unlock the objective Curator Class VI, I noticed that the objective tracker was showing progress on the 'get three stars on all museums' goal as '-1/3' (negative one out of three). After getting my current museum (Pebberly Heights) to three stars and returning to the main museum map, I saw that two of my other museums, Passwater Cove and Bungle Wastelands, had reverted to one star rank, instead of the two star ranks I'd achieved before (and needed to achieve to unlock the current Curator Class).

I haven't yet tried to re-enter either of the reverted museums to see what happens. After attaching my current save, I will likely attempt it to see if doing so restores the two star ratings or sends me back to an earlier phase (or something worse).