Why does my master page move when the page shuffles left/right?

I am laying out a book in Adobe InDesign.

Context: Since creating the original book document, I have changed the page size from 6x9 to 7x10. All my master pages are of the new size (7x10). (I have a feeling this relates to the problem somehow.) I have page shuffling turned on.

When a page shuffles from left to right or right to left (due to an earlier chapter changing the number of pages), the master page moves -- and is now misaligned with the page.

Before shuffle:

After shuffle -- note how the master page no longer overlaps the real page correctly (which messes up a bunch of other things).

If I right click on the page and re-apply the master page, it's fixed. This is a pretty long document and I can't do that for all the pages. (I could one time -- but doing that every time a page changes is going to cause a lot of issue.)