I suspect the issue lies in the introduction of firejail for jolla-gallery. I presume, xdg-open hasn’t been (fully) adopted for that and causes this issue.
I have filed a bug report (both here in the forum as well as internally with jolla) for that: [4.2.0.21] [X10 II] Video player overlay missing from videos
Another interesting issue that arises from this is that you can’t open videos if jolla-gallery is running in the background. You simply get the “open successful” banner on top, but nothing happens. I have filed a bug for that as well: [4.2] [Xperia 10 II] Jolla-Gallery won't close properly / can't open video from file manager