`Had this problem earlier today. Could reconnect the mouse by removing the Bluetooth connection and re-doing it, but that should not be required.

Turns out that there is a trust issue with Bluetooth, luckily, someone has already figured out the fix, which is easiest to do via the command line. Its all documented here - https://ubuntuforums.org/showthread.php?t=2390542 & the original here - http://mielke.cc/brltty/doc/Bluetooth.html#for-bluetooth-version-5.

Weirdly, I didn't have the same issue with bluetooth headphones, only with the mouse.