I spent 2 hours to find out why a page disappeared - out of a sudden.
The reason was an orphaned text file which contained a Uuid as single information. During a deletion of the image jpg file the corresponding text file was not removed - for whatever reason.
As a follow the folder with the text file wasn’t interpreted as album page any longer (which it was normally), but as something else, so the default.yml blueprint file was taken. As consequence the page disappeared from the album list…
Just for your information.