Why can't I type on my pages?

Now suddenly (much of the time), I can't type on a page after creating it. I have examined the master pages they're based on, and the problem appears to be a bug where InDesign loses the assignment of the "primary text flow." I have facing pages that intentionally do not flow between each other (yes I have checked both directions).

But InDesign will not let me specify the primary text flow on each page. If I set the one on the left, it loses the one on the right. Then, depending on which side my next new page falls, I may not be able to type on it. The problem appears to be due to the "primary flow." In the screen grab below, you can see that the "primary flow" toggles between the left and right pages.

Neither of the panes is locked.

Now what?