We can't presume something is easy if we do not know the code. Training skill arrows pop at training and stay there before the next training happens. With experience, the pop happens during the game. We do not know how complex the training code is. What get's checked and how are arrows generated.
Agree with you, I've never understood why every skill has an arrow but experience.
Game shape does not have an arrow aswell. Although it pops/drops at training. So there must be a different kind of calculation for it. Experience on the other hand does not have a drop at all. Perhaps game days are a huge strain on the servers and running that check, might not be optimal.
If needed, people might check for the pops manually, as you know, when the pop can occure. I check for exp pops after every training.
I think this suggestion has some merit though. If it is relativly easy to implement, it would be a good addition. What we need to thing about is, how long will the arrow stay? It pops after a game, get's removed after the next game (regardless if the player plays or not, because training arrows stay until the next training update)?