I have a similar problem.
In my case the PowerBeats Pro connected to my Mac just fine. I then disconnected them and connected them to my iPhone. SO far so good. I then wanted to go back to using them on my Mac. Assuming that they are like other Bluetooth devices that can only connect to one drive at a time (afterwards I read somewhere that they can connect to two drives and stay paired with them, but I have not gotten that to work for me) I unpaired it from my iPhone and tried to pair it back to my Mac.
No matter what I do it will not pair back to my Mac, even though I had paired them successfully an hour before.
I see them show up in my Bluetooth panel, and I see the connect button but each time I click on the connect button I get an error message that says connection-rejected.
And they will not pair with my Apple Watch Series 4 at all. I don't know if there is a different procedure for that.
I did do the firmware update yesterday (it is a slow process, the progress bar barely looks like it was moving but eventually it did complete the firmware update but it did not fix the problem.
https://www.beatsbydre.com/support/how-to/firmware-updates-beats-updater
I tried to reset them but that made no difference.
I then found a post online that showed how to delete the preference files, which I did and I rebooted my Mac and the PowerBeats Pro still have an issue.
In my case I suspect that the problem is a preference file that is noting that the Beats were uninstalled and somehow it remembers that and tries to not connect to them again. My suspicion is that the file is corrupted and that is the problem that many of us or all of us are experiencing, but I have deleted all the Bluetooth preference files I could find on my Mac and that has not change the symptoms at all.
I posted all this information on the support page for Beats but there is nowhere to put your email address so there was no way for them to get back to me with a fix.