I'm pretty active in the av1 community. Most of us who use av1 encode our own from the raw blurays or high quality remuxes. Besides the av1 content on public trackers, I think I saw a group called onlyfaffs and another one called WhiskyJack who were both putting out some av1 content, but imo, their filesizes are too large, so I avoid them if possible.
The other thing to worry about is that most people who use av1 also convert the audio to opus. Fully opensource codecs and all that. The issue is you don't know what bitrate they are using for their conversion. So audio is a concern too when downloading av1 content.
Nothing. It’s just that you don’t see people use flac very often as an audio codec for movies or TV series. At least I haven’t seen it very often.
The only issue would be if you were trying to transcode Dolby Digital to flac. That is not a transcode you want to do.