Whenever i play as a level 4 champion, and play an S game and hit level 5 in the same instance, i hit level 5, then additionally i'm told i earned a mastery 6 token. But when i look in my hextech crafting i don't see it. This has happened to me twice now, which is very upsetting.
You aren't intended to earn a mastery token in the same game that you reach mastery level 5, only for games afterwards. The notification that popped up was saying that you are now eligible to earn mastery tokens for that champion, not that you have actually earned one, though the two do look similar.
i earned a mastery token 6 at Volibear while playing ARAM and it has not appeared at the loot :P I was so dissappointed...
