I submitted my Bluetooth disconnect issues, with system logs, to Apple engineering a few days ago, via Feedback Assistant app for Apple Developer & Public Beta testers. I'm going back to Asus dongle for now my 5.0 dongle is on its way, and hopefully that will work well AND allow the Apple Watch to login to the Mac Mini (that seems to be the only feature missing from the 4.0 dongle). I hope Apple reads these threads, but if the discussion forums are just for users to help each other, I don't know the best way to report this or get it fixed. The Asus doesn't seem to vary the power, which results in a steady link quality? If this IS software controlled, it seems it would be relatively easy to add a software switch that lets the user choose to maintain the power constant, or choose save energy mode to vary the transmit power and save energy (like while on battery power on a laptop). So, in my mind, (and I'm a physician not an engineer), either the Broadcom chip itself, or the software controlling it, varies the transmit power to save energy (which on a laptop would make sense, but on a desktop unit less so), and that drops the connection. Same equipment, environment, everything- only difference is an external dongle by Asus and the internal Broadcom bluetooth chip with internal antenna. Also note that the Transmit power sawtooths all over the place with the ASUS BT400 dongle, the link quality and transmit power are 100% all the time no sawtooth pattern on the transmit power, and completely flat at the top for link quality. When the disconnect-connect-disconnect-connect happened, the link quality dropped to 0. Attached is picture from Bluetooth Explorer: So I am not an expert at this tool but look at the link quality.

Asus usb bt400 driver error