While checking earlier threads about insets and unwanted spacing, I came across references to PgfLocked as a property you need .mif to get to. It wasn't part of my problem, but I'm curious: what would it have been doing, if it had been there?
I've speculated before that PgfLocked is a structured tag that escaped into unstructured Frame. There's also a TextInsetLocked tag that is similarly mysterious.
The thing I would have supposed these are used for, they are not. They appear often in my test documents, but are set to No.
Cross-References by <$paratext> and Text Insets don't just have tags pointing to the text source, they also have a local copy of the entire source text, as of last Update, at the insertion point. This clone text needs to be protected against internal modification, i.e. when you attempt to select any of it, you select the entire object. I would have thought that the Locked Yes tags might be how that is done. Testing it just now; they aren't.
As to why a PgfLocked Yes often appears in documents with extensive Text Insets, such as when the entire Flow A is an inset, and you delete it, I'm thinkin': bug.