This didn't happen with affinity v1, at least not with the version of it I was using.
With v2:
When I add or remove one page somewhere in the middle, or move one page to a different place, it adds master pages to all pages following the place where the page has been added / removed / moved to. Meaning: following downward from the place of removal/addition, all formerly right pages that have now become left pages have the text frames and images of my "left master page" on top of that page's text frames and images. All formerly left and now right pages have the text frames and images of my "right master page" on top of that page's frames and images.
This only does not happen if
a) I have changed nothing about the page yet. Even if I have only typed in some text in a text frame, without moving the frame from its original place, that page's master page will be placed on top.
b) It also does not happen when I add / remove / move an even number of pages. The problem seems to be when a formerly left page is turned into a right page, and the other way around.
This is a huge problem. If I have to remove a page somewhere at the beginning of my 100 page document, I then have to go and delete those additional frames and images from all the 90-something pages following fromt there.
Am I doing something wrong, is there a setting I need to change, or is this a bug? If yes, please fix it soon - I cannot do bigger projects with publisher if this keeps happening!