If you play a game, then die, then go play another game in the same plane/vehicle while the first match is ongoing… ANY and ALL RP you earn in that 1st match will be deleted for that vehicle (referring to modification RP, not tech tree RP) EXCEPT for the RP that went towards the current modification you were researching. This is why you will often come out of another battle with the modification screen up asking you to select a new mod, but with 0 extra RP to distribute.
This needs to be removed. It serves no purpose and only slows progression. It’s also not a bug. Gaijin confirmed this in a bug report.