What you're seeing is the Movie plugin try to open the file, but fail (due to the splitters/codecs not handling it), and then Opus falls back on another plugin.
It's not an issue with the Movie plugin itself (which hasn't changed between when it worked and when it stopped working) but with the 64-bit FLAC splitter/codec or corresponding registry data on the system.
It's possible VLC modified the registry when it was installed. It's been known to in the past.
Note that on a vanilla Windows 10 install, testing with Opus 12.14, FLAC does not play by default in the Movie player. For that to have worked, you must have installed something extra (something that installed a 64-bit FLAC splitter/codec). If you can work out what that was, then reinstalling it would probably fix the problem. (Another possibility is a recent Windows Update broke something. The test machine I'm using is fairly up to date, so I haven't tested whether it worked out of the box on older versions of Windows.)