I've used PowerDVD 15 Ultra (Version 15.0.4630.62) for Blu-Ray disc playback since it was new. It always worked fine. Sometime in the past month (not sure exactly when), Blu-Ray discs (and essentially all HD and 4K video files) stated playing with extreme brightness. Windows 10 1903, nVidia GT-730, Samsung U28E590 monitor. SD video and DVDs play fine. The excess brightness is well out of range for proper adjustment with TrueTheater and other settings in PowerDVD. Blacks become a mid-tone gray.

If I use nVidia Control Panel and disallow apps from using their own brightness settings and adjust the video instead using the nVidia Control Panel sliders, I can get blacks to be black and reasonably good color and gamma on the video in PowerDVD. But those settings, of course, are wrong for all the other video apps I use (most often, the Windows Movie & TV app).

The Windows 10 upgrade to 1903 took place for me on 7/27/2019. But the earliest System Restore point I have available is 8/5/2019. I tried it, but the problem was still there and I undid that System Restore.

The nVidia driver is 23.21.13.9125 dated 3/16/18, and I assume it's been there since roughly then.

Any ideas on what may be causing this sudden recent change to excessive playback brightness?