I've only had this problem for a few weeks where I put both of them in and it registers one side. This is on my iPhone 13 mini. I usually have to play around, either rebooting the phone or pairing with my iPhone 7+ to get both back to working again. One thing I've started to do is to listen to music with my iPhone 7+ as it doesn't have this problem.
I am updating the iPhone 13 mini to iOS 16.3 to see if that fixes it. It's possible that the Beats Fit Pro needs a firmware update too.
I am updating the iPhone 13 mini to iOS 16.3 to see if that fixes it. It's possible that the Beats Fit Pro needs a firmware update too.