Never seen this before, in ~30 months of using PrP so far:
I was doing a routine export of a multicamera sequence (matching sequence settings). And the exported video switched (appropriately) between cameras accordingly per the source timeline, but only up to ~26 minutes in. Thereafter, all the camera selection changes were just ignored, so that the exported video is, from ~26 minutes on (~6 minutes worth), all from only one camera. Upon discovering this, I checked the source timeline, and all the camera change edits are in place, as they should be. So, the camera switches were not reverted by some weird accident. Rather, the problem was solely in the export.
I'm not posting this as a question, since I'm not really expecting an explanation. (Feel free to surprise me, though!) Mostly, I just want a record of this weird little bug, in case it shows up again.
The worrisome aspect here, is that this export was rather long (~32 minutes), and I was not planning on reviewing every second of it, post-export. (The plan was to just re-import it into another PrP project file, to append to some separate work, to make the final complete timeline.) I only got lucky (barely) to have noticed the problem, when setting chapter markers for the final project, and the camera angles struck me as looking wrong.