added a comment - - edited
Note that the test program is decidedly simple to facilitate automated bisecting.
It's the simplest program I could come up with quickly that failed in a manner similar to the application the problem was originally seen in.
The application uses QPP for path masks, and allows the user to interactively edit these (think PhotoShop).
The problem surfaces when the user unites two path masks: the path masks become polygon masks; the result cannot be edited as a path anymore, only as a polygon.
QPP may be the wrong abstraction for implementing user-visible path masks, but given that Qt itself uses QPP as clipping masks (on QPainter), and it worked in 4.6.3, I think this issue warrants consideration.
So, yes, the report is misleading. It's really about the fact that uniting two bezier-containing paths now yields a polygon-only path.