Some could be:
* Kylo has not been the Master of the Knights Of Ren for long.
* There are multiple "Masters" or Kylo and Ezra are co-masters
* Ezra was injured in a duel with Kylo for the title and is now returning.
* Ezra is a darkside user but not a KOR and has a very prominent role in the FO
* Ezra is stationed somewhere in the Unknown Regions (possibly being sent on a mission by Snoke) with little contact and is only just returning and becoming aware of Kylo
* Ezra has been frozen in carbonite
* Ezra heads a splinter dark side group either independant of the First Order or within the FO but in competition with the KOR
* Ezra just doesn't have an immediate issue with Kylo being the Master Of The Knights Of Ren
* Ezra has another title within the KOR above Kylo. Maybe Ezra is even a Sith apprentice and Kylo as Master Of The Knights Of Ren wants to become one.