I'm having this issue. And this is the only reference I can find online:
beats flex and apple watch problem - Apple Community
In my case, it happens when I'm using Spotify.
beats flex and apple watch problem - Apple Community
The sound on my new Beats Flex goes out when my apple watch shows a notification. My apple music library is on my iphone 11 and I keep my apple watch on "now playing" to control my music . I also use the apple watch's exercise app simultaneously while running and listening to music which on other bluetooth earphones and with apple airpods worked without problem. There would be a momentary interruption of the music during a notification then the music would start back up on its own.
With the Beats Flex, every time my watch chimes a notification on the exercise app, the sound goes out and will not come back on unless I drag out my phone and hit the play button. Its like the play button on the earphones gets disconnected even though the music app show that the beats flex are connected It is very annoying because I get multiple notification while running and I hate having to fool around so much getting the sound back.
Please help if you can.
Thank you!
In my case, it happens when I'm using Spotify.