I used to have a random mission debrief under avenger bug every 2 - 4 missions. However, after a lot of research I found out that XCOM 2's issue with memory leak is the cause of this. Also, this memory leak gets worse the more mods you have. So usually it isn't a particular mod's fault. (Unless it's code itself has loads of memory leaks)
Okay, so here's how i fixed it.
I used an external program to clear my RAM of garbage every time I start a mission. The program i use says that I clear 1 - 2 GB of garbage every time I clean before the mission. (Do it before the mission or it causes errors for unknown reasons. So don't clear the RAM at the end of the mission, but do it just before the mission starts) By doing this, I've played the game without the so called bug for about 15 missions now. Alt tabbing to press 'clear RAM' is far more faster for me than restarting XCOM 2 because I have a very long list of mods. (About 50 which are mostly cosmetic stuff.)
So this was the solution for me. The exact name of the RAM clearing program I'm using is 'Advanced SystemCare 10' It has a little program that clears the RAM for you. Although I think it works with just about any program that clears the RAM.
Let me know if it works for you too. Cheers.
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)
EDIT: A stupid typo that I just realized.