I. Elmelyan
Le gardien avait un potentiel de 79, a perdu son potentiel et sa note, et les points de compétence ont été distribués de manière incorrecte.

What was the lineup in the match on 30th of July? Based on mostly fitness, it looks to me like Quijada was GK, while the player in question played as an outfield player and was subbed out for a winger about 15 mins before the end.
I'm not exactly sure what are the rules regarding GKs playing games in other positions as I've never considered it myself, but that could be a reason why his potential was "reset".

If the player positioned in GAC does not play the whole match in this position, his potential is reset.

Premièrement, je ne le savais pas.
Deuxièmement, lorsque j'ai essayé de faire revenir un gardien à son poste d'origine, le changement n'a pas eu lieu.
Troisièmement, voulez-vous dire que si vous jouez un gardien à un poste comme AML et que vous le changez de poste en milieu de match pour AC, il recevra un zéro ? Ou voulez-vous dire que le gardien de but est sifflé s'il entre dans une position autre que GAC ?

No, this only concerns changes to the GAC position.

A goalkeeper can be a DLC by playing every 2 days or a goalkeeper by playing every day.
He must not do any other job before or after the match.
Why not? Because he would take too many points.
If you have a goalkeeper in training who plays dlc every 2 days without moving, that's ideal until the end of his training.
Otherwise, he has to play goalkeeper every day.

Before and after, he can change position without losing, it is only during that he must not

I don't think you can alternate goalkeeper positions.

Let me explain what I understood: a goalie
in dlc which day every 2 days = a guard who plays every day
So on you put it dlc then the next day keeper it will take more points than it should take in reality.
To simplify, he'll take 2 points as a day keeper or 4 dlc points every 2 days.
If you put him 1 day as a keeper and the other as a dlc, he'll take 6 points in 2 days instead of 4.

He won't be able to play every day because he'll get a lot more tired playing DLC but, yes, he'll play a few more games than if he only played DLC.