I had a part that seemed to always have one of the text frames at the top of the page squished. I’d fix it in the master page. Then all the other parts would show the same frame stretched too low.
Mystery solved: if the margins on some parts are even a little different than margins on other parts that share the same master page, they will respond differently when you push out master page changes with the apply button. And depending on which page is showing in engrave mode, the master page when opened for editing, will display differently.
I don’t know if this is intended behavior or an unintended consequence.
Put this in the “helpful to know” category.