In rare cases I get a freeze like this with the stock music player in connection with Bluetooth playback. A simple reboot always fixed this so far.
Music is playing fine without a Bluetooth connection for me, so I blame it on Bluetooth being the mess it is.
I’ve used LineageOS on the Fairphone 2 since 14.1 and can’t complain at all about the stability of it.
As LineageOS 15.1’s days on the Fairphone 2 seem numbered already, perhaps you want to try whether the problem persists in the upcoming LineageOS 16.0 based on Android 9 …