Another user had an issue that was related to a gallery plug-in they were using, so I imagine it is something similar.

Glad you were able to find a solution, however inelegant… :-)

So, my guess is that it has something to do with some other plug-in,maybe try Deactivating other plug-ins one-by-one and refreshing the page, see if the page class suddenly jumps in as expected?

But, please, if you ever do find anything, please let me know so I can add the solution.

Thx again,
Atg