It appears my problem is not a new one...
My music from Spotify plays well normally on my Forerunner 245 Music thru headphones (JBL Go Air Sport+), but starts glitching/cutting out when I try to use GPS to track an activity. I've tried pausing/playing, using different headphones/speakers, forgetting/re-pairing headphones, system update, and a full reset of the watch. At this point, there's no way I'm paying over $100 for a refurbished one just to have the same issue.
The music capability is the main reason why I wanted a Music model... what's the point if it doesn't work while you're running?!