WARNING: USE AT YOUR OWN RISK, YOU WILL BE RESET.
I use the XP lobby method from 1st Prestige level 18 to 4th Prestige level 1.
All last night and tonight I have been grinding from 4th Prestige level 1 to 5th Prestige level 49.
I logged out to get some food and have a break, I logged back in and played a game as normal.
I was then removed from the lobby and it displayed an error about connecting to servers.
I then tried again and a message popped up saying I have leveled up in a modified game therefore I have been reverted to my previous rank.
Which means not only did I loose the XP I boosted using this method,
I grinded 2 whole prestiges almost during this Double XP weekend after boosting to 4th, and lost ALL of the progress.