My setup is Forerunner 645 M, software version 6.2 with the Aftershoks Aeropex Bluetooth headphones.
The issue is that the volume + doesn't work. The Aeropex volume works perfectly with my phone, just the watch seems to be a problem.
(I see that this issue has also been reported for the FR945, but figured it would be a good idea to also report it for the FR645.)