I got prescribed beta blockers for migraine, half dose, but gave them to a pharmacist after reading about them and talking to doctors, nurses and cardiac technicians. TBH scared me so I decided to manage it another way.
I suspect it's just doing its job but since you don't need it for that job it's not what you want hence instructions to go back to gp if it goes below 50bpm. Suggest you visit your gp about it, at least the peace of mind.
From what I remember, for migraines the start you off on half strength then adjust as needed. Sounds like your dose needs adjusting. In my case the consultant had to lead the treatment because GPs can't prescribe beta blockers but can do repeat prescriptions. Whatever the case you tend to go through the gatekeeper, I mean gp.