i've just got the same "problem" when trying to apply the Master-Pages spread properties to all as well as a specific set of pages, the dimensions are not being adjusted accordingly.
This also happend when just applying "clean" Master-Pages, on which only the Dimensions have been changed.
But since this is the first bug i found within 1 week of regular usage of Affinity Publisher I'm quite happy so far
keep on going
Welcome to Affinity Forums
Sorry the delayed reply. This is something the dev team is already aware/looking into. It's an area that still needs improvements and may see some changes in future updates.
Thanks for your feedback.