In my next-gen project (PX) board I created a column "IN REVIEW".
If I now add a filter that filters by status = "IN REVIEW", the next time I enter the search results for that filter the status drop-down contains both "IN REVIEW" and "In Review" as selected values, and a red exclamation mark appears next to the drow-down with a tooltip that states "Status In Review is not valid for your current project".
The JQL expression is simply:
project = PX AND status = "IN REVIEW" ORDER BY created DESC
It seems to me that the drop-down is trying to populate itself with everything that matches "IN REVIEW", ignoring case, but in the context of project PX only the uppercase identifier is valid.