drops a selected class after several options

There seems to be a class just not displayed on each schedule even though it is available during that timeslot.
First everything is fine:

Then a class disappears even though it could fit at that same time slot in that particular schedule displayed.

Then it returns again:

Settings as shown, none of the class options are messed with, just added a time for not having class, clicked show classes, then scrolled down and increased page size to 50 and noticed the class had disappeared on some schedule options.