Using different master/parent pages in conjunction with primary frames

Lately I've been running into a snag I don't remember experiencing before, though I've been using the same method of layout for almost a decade.

I am a book designer. I always create my InD doc with a primary frame that will be used for the majority of pages. Then I create several master/parent pages for different circumstances such as the beginning of chapters or divider pages for "parts."   The chapter-start masters usually include a partial page sink of 6-10 picas.

The problem (again, this never used to happen, and I don't know why it does now or what has changed) comes when I need to apply the chapter-start master to a page in the layout. It applies to the page but NOT to the primary text frame that is on the page. Instead, it places itself as a second text frame UNDERNEATH the primary text frame.  I am not getting the effect I used to get, which is to apply the master page text frame attributes (e.g., page sink) to the primary frame already sitting on the document page. Instead I just end up with two text frames, and my text ends up in the incorrect one of the two.

Yes, I could create an object style to apply to the primary frame when I get to the beginning of a chapter, but there are master items I want to add to these pages, and sometimes using an object style for the text frame is only a partial solution and leads to a multi-step process to get the rest of my page attributes in place.

As I mentioned above, this is a problem only recently. I know for a fact that this never used to be a problem. I suppose there's a possibility that in past I somehow placed the chapter-start master on TOP of the primary frame that is already in place on the document page, and if so, I probably would not have realized I had two text frames on the page. But I never noticed doing so, and even if I had and even if that worked without my realizing what I'd done, why has THAT circumstance changed?

Any thoughts will be appreciated!