The mouse suddenly getting out of the game problem

To be honest you should first and foremost raise a ticket with support.

Secondly if your game keeps tabbing out then that's proably a problem in your PC itself. A programme is then constantly trying to go into the foreground, try checking your Pc for malware, Malware is the only type of software I know that does that. You can also see in your system tray what programme is active at any time so you should se something flashing there asking for attention (adobe trying to update something is always a great example of this).

Hope this helps.
Cheers

@callmebackdraft @Kikimacia if this is the only game you run in fullscreen borderless mode then it might cause problems you'd otherwise never have. I've been running borderless windows mode since I got a second monitor years ago. (it works so much better in all games to seamlessly hop to the internet and back into the game)

I've had problems like this with Adobe updater, i've promptly removed all adobe products from my PC and the problem was solved. I've had customers who've experienced similar problems with malware installed that tabbed the screen often, for this I installed malwarebytes and removed the malware and the problem was solved.

I feel that the problem isn't so much in the borderless fullscreen mode as it is in software that so badly wants to move into the foreground and thus tab out the game.