Changing the amount of slides does not work after the first time

Summary

If custom slides for a presentation have been edited before, then changing their number no longer works when editing again.

Steps to reproduce + Current behaviour

1. Log in as somebody who can edit session slides
2. Create/go to session that has slides
3. Custom slide amount is currently (presumably). Add some custom slides, then remove some to check that it works, but ensure that there is at least one custom slide left.
4. Save.
5. Go back to editing slides and try to add or semove some. Both do not work, though the number changes in the input field.
6. If you save and go back for the third time, the number is also back to what it was after first saving.

Expected behaviour

Editing the number of slides should change the number of slides (not only the integer in the input field) no matter when you edit it, and should persist that change.

Screenshots