ok I thought you were using a trackpad in which case 3 finger tap is middle click which is paste, but since you are not using that it may be something else, something you may want to do is to disable your trackpad entirely while using your bluetooth mouse
I too have a similar issue with the mouse jumping around every so often and it is due to the touchpad on my acer f5-571 laptop. the check for diable while typing makes no difference but i just have learned to adjust my position when typing to prevent it. Not ideal but its ok, or i use an external mouse and disable the touchpad. It is for me because it is very a sensitive touchpad.
I have tried a bluetooth mouse and had a similar experience but that was more down to the surface i was operating on, bit of experimentation with different pads and surfaces solved that. the best is still a single dark colour, nice and flat, surface, although some small contouring seems to make performance slightly better.
Just my experience
Now, it has started disconnecting a blockchain app every so often. This app has been working fine for months. Some recent update has broken it. 18.04 is trolling me hard! It has finally come up with a deal breaker. I have been postponing the return to 16.04 for the sheer amount of labor involved in getting all my apps and customization back.
The panel sometimes forgets the positions of items on reboot. Sometimes it will take stuff from display1 and put it on display0 and vice versa.
Kdenlive (among others) does not pick up the Ambient Dark theme. I cannot see the scrollbars, I cannot see the button icons of toolbars, I cannot read text in some places before highlighting them. Couple of decades in and we still cannot get a theming framework that works consistently across all apps----this is the Linux desktop.
Sometimes the file manager will tell me I do not have permissions to rename things that I am the owner of. Only a reboot fixes this one.
I am sending system error reports pretty much daily, sometimes multiple times a day. I rarely manage to go more than 48 hours without needing a reboot.