I am having the same issue. Did you every get a real solution? Here's what happened in my case: I bought a tp-link Bluetooth 4.0 Nano USB adapter and plugged it into a USB slot in the Dell monitor into which I had my MacBook Pro 16" (2019) plugged via USB-C/Thunderbolt. The adapter took over, and now my Magic Mouse 2 won't work on my MacBook Pro unless the adapter is plugged into the monitor. EVEN THOUGH the Bluetooth module in the MacBook pro recognizes the Magic Mouse 2, it won't actually connect. NVRAM, SMC, restart, restart in safe mode, device removal and re-addition, etc.—I tried it all. And I still don't have a mouse that will work with this computer. However, I am fairly certain the issue is with the MacBook Pro and not the Magic Mouse 2 because the Magic Mouse 2 works with two other Macs I have.