They're having issues with rendering subtitles, not displaying the video. Even if they were to avoid AV1 and instead get an equivalent HEVC release they would have the exact same issue - not rendering the same subtitles correctly. There's nothing special about any particular codec that allows their media player to render subtitles correctly or not. Those subtitles either work or they don't. On a side note: Developers of VLC are responsible for the most ubiquitous AV1 software decoder in use today: Dav1d. It's no surprise that AV1 displays properly in VLC - in spite of all the issues we've come across.

VLC users are for whatever reasons often running out-of-date software or are unfortunately on buggy versions. This is the reason why most of the people on the scene recommend MPV over VLC without providing details or additional tech support. It's understandable why - the amount of complaints we've seen regarding subtitle rendering or audio playback on VLC vastly outnumbers the complaints we've seen with MPV (we've seen only a handful of complaints and they have nothing to do with playback).

While we don't subscribe to telling people what software they should be running - by all means choose what works best for you - there has to be a point where you pause to consider "Is my media player up-to-date?", "Is this the best solution?", or "Should I try something else?". Consider expanding your options instead of compromising on them. Again, use what you want but don't be surprised if your experience doesn't meet your expectations.

Have a nice day,
-Sav1or