Not receiving mastery 6 tokens on summoner rift

So I have a theory that there is a limited number of mastery tokens that you can have in your crafting inventory at once.Reason being, I have gotten s ranks with champions that I already have mastery 5, but not receive the popup thst i got a token or even seen it in the my inventory. Can anyone confirm that this is intended or if it is a bug? I've missed out on a lot of tokens for champions if it is and that makes me sad. :/