Try upgrading to Monterey 12.3 Beta 4 which includes a fix that restored RFCOMM connectivity to our custom hardware device. Hopefully it does for you as well.

As a follow-up on my other answer: As of Monterey 12.3 Beta 4, RFCOMM handles for our device are being properly returned and connectivity is restored within our application. As an additional note, calls to IOBluetooth.recentDevices which we were leveraging are not currently working (at least for us). We have reverted to using IOBluetooth.pairedDevices() instead, which provides similar information.

We're facing the same issues with our commercial application -- something is wrong with CoreBluetooth since the latest update. We are also submitting tickets for this on our side.

Our production application has also been unable to create Bluetooth socket connections since the last update. We have validated this issue with two independent API for Bluetooth connection -- all of which are fully functional up to and including Big Sur. Customers having upgraded to Monterey are all unable to connect their devices to our desktop application. This is critical and we are waiting for a fix.