I think I've found a workaround for this problem (and don't think it is Parallels that is at fault here). After doing some digging around in Apple's Bluetooth Explorer development tool, I discovered that macOS Catalina is actually attempting to use my external bluetooth adapter/dongle as the Bluetooth host controller for macOS. Because the bluetooth adapter is "in use" by macOS, it can't be mapped to the VM. This did NOT used to be the default behavior for macOS. A little searching around and I found you can instruct macOS to not use external adapters as host controllers by typing the following in terminal.
Code:
sudo nvram bluetoothHostControllerSwitchBehavior=never
After that, restart your system.
After a reboot, I was able to map my Bluetooth adapter to parallels like I used to be able to. Hope that helps you all!