I have had the same problem in Arch.
I’m not sure exactly when it broke or with which package upgrades… helpful I know.
Doesn’t matter whether hardware or software decoding.
I’m on vanilla Arch as well and I think I finally found the reason for this.
The upgrade to Qt 5.14.1 came with some pretty hefty changes for how Qt managed OpenGL and Qt Quick’s scenegraph rendering. Most of it is opt-in but some have had breaking effects on apps using OpenGL. I eventually found this Qt issue which led me to believe those changes were interfering with PMP’s OpenGL rendering setup with libmpv.
Long story short, the prescribed workaround was successful for my setup. I changed my command line to plexmediaplayer <plex_command_line_opts> -- --disable-viz-display-compositor. I now have a working plex-media-player again on Qt 5.14.1.
Quick addendum, I’ve tested a patch that adds the Qt flag to the application main, so you don’t need to actually change your command line. It’s here for those interested, and I’ve posted it to the AUR thread.
Final addendum, the AUR maintainer has accepted my patch and several folks confirm they are working again. I’ll open a pull request against the Plex upstream repository, but I’m not hopeful we’ll get it adopted as they’ve rejected these sorts of fixes to keep current with Qt in the past. I’ll likely continue to maintain a fork for these purposes.