I've been using Poweramp on my android phones for the best part of a decade and I've had a Garmin Fenix 5S for the last year or so.
Until recently I could use the music controls on the watch to play/pause and skip forwards and backwards on Poweramp V2 without a problem, which was great for running and cycling. Since the V3 update landed though, the music controls on the watch have stopped working with Poweramp.
Other than this issue, the watch is working absolutely fine and even the music controls do still work with other apps (Google Play Music, Spotify)
Whilst the watch's play/pause/skip controls no longer work with Poweramp, the watch will still display the title, artist name and how long the song's been playing, and the volume controls still work. Also, if Poweramp isn't open on the phone (but it's the most recent music app I've had open) then pressing play/pause/skip on the watch's music controls will open Poweramp.
If it helps I'm using build 816 of Poweramp on a Samsung Galaxy S7 running Android 8.0, and the watch is running v12.3 of its firmware. The music controls stopped working the day I got the Poweramp update, and I'm fairly confident I didn't get any android/garmin updates that day.
To troubleshoot, I fished my previous smartphone (Sony Z5 compact, still running Poweramp 588) out of a drawer, and synced up the watch. The watch's music controls still work absolutely fine with the old version of Poweramp.
It's a real shame because I actually really like the new Poweramp v3 UI. I don't want to lose the ability to control music with the watch though so I guess I'll have to use other music apps until this is fixed.
If anyone knows how to fix this I'd appreciate it, but if not then at least it's been flagged and can hopefully be fixed in a future update.