When im calling await api.League.GetChallengerLeagueAsync(Region.Euw, Queue.RankedSolo5x5); the enties modal looks like this:

As you can see leagueName is null for all entries. I think this is because the Riot api defines this property as summonerName and also summonerId, which is missing. Changing the region or queue doen't change the result.
I think that this might be a bug since the LeaguePosition model doesn't corresponds with the RiotApi model.