Hey, not my experience but this happened in ome of our leagues.
Basically, to display player's stats in league leaders at all, that player must have minimum games played in league for your team in this case. Stats displayed are from previous team and that's why they don't change with matches he played for you.
If you open your team statistics, you can see that mentioned player has 3 gp. For some reason this counter resets if player changes team, even in same league.
It also impacts your merchandise of course. For him to have "normal" merch values, he has to play minimum games required for him to be in leaders tables. Same goes for bonus for being top league leader