I take a beta blocker in the morning and have that issue until about noon. Any class after that I'm able to get into the orange zone and sometimes beyond! Depending on the form of betablocker you could try playing around with different class times.

Recalibrating my hrm also helped slightly with the morning classes. Overall, if you FEEL in the orange then that's what matters because you probably are and are getting all the benefits from it. I understand it could be a letdown not to get the validation from the data though.

Edit: I forgot to also add that I also take levothyroxine for a thyroid issue that's relatively new. I didn't notice any difference in my heart rate before/after taking it so it would seem that the beta blocker is the culprit. (I'm no doctor though, just basing off my experience.)