Mastery 6 tokens

Hi, Twice now I have gotten an S- in a ranked game on Lux, and not been given a mastery 6 token, despite being mastery level 5. I already have two of the tokens, but have missed out twice on getting the third one, and it's super frustrating. I believe this started happening since the most recent major update, where the login screen has a whole new white theme.

Is it possible this is a bug?

Any ideas on the issue would be greatly appreciated!

Cheers, Lemon