ImprovementRank info now notes the current Green I cap
The rank explanation now states that promotions currently cap at Green I, with higher ranks coming later.
The rank system screen now notes that promotions cap at Green I for the current season, and that higher ranks will unlock in a future update (in all 16 languages).
Several players asked whether being stuck at Green I was a bug — it is by design for now. Higher ranks are on the way.