I don’t know if you have noticed, but if you are research vehicles or modifications and have only a few hundred RP left to complete, then start the first game. If you quit the first game before it ends and then start the second game, you will find that all the RP gains from the second game will disappear.
As far as I know, this bug has existed for at least 2 years and has not been fixed. It is very likely to cause players to lose 10%-30% of their RP earnings. I even suspect that Gaijin is aware of this bug but is deliberately ignoring it