Bowers & Wilkins P5 wireless headphones on OSX causing problems for other bluetooth devices
I had a similar issue a few months back but it resolved itself when I upgraded to Sierra. Unless there's a reason for holding fire with the upgrade, I'd suggest giving that a go to see if this resolves the problem. 

Do you have other Bluetooth headphones/devices, or perhaps another laptop to test with? Would be interested to see if a combination of other devices had the same affect on your Microsoft mouse. Although the disconnection only occurs when the P5 Wireless are connected, it sounds to me as though this could be a red herring!