At the bottom of each page, there is a check mark to "allow in mastery paths". I've had several teachers check this accidentally. As a result, the page was unavailable to students.
This behavior is described on this page: How do I allow a page to be a MasteryPath module item?
The problem with it has been pointed out earlier by Damion Young in his idea Rename "Allow in mastery paths" to "Show only in mastery paths".
Renaming the check mark would certainly be an improvement. But in my opinion, the check mark is unnecessary. I would suggest to remove the check mark altogether and allow all pages in mastery paths by default.
Only when a page is actually added to a mastery path, it should become unavailable to students until the required score on the conditional quiz is reached.
This would prevent unnecessary confusion about pages not being visible to students and it would make the procedure for setting up mastery paths leaner.