Stephen, I was able to duplicate your problem on my ipad Gen 5. I was able to start and stop the first flow in several places. Then I started play near the end of the first flow, it went into the second flow, and I could not stop the play. Then it crashed. Note that all of my troubles were in Write mode, page view.
After reopening the file, I’m able to start and stop play if I highlight the first note in the 2nd flow.
I tried it in Galley view by selecting the last rest in first flow and pressing start play triangle icon. Once it got into the second flow the stop play square icon didn’t do anything, and Dorico crashed partway into playing the flow.