Custom Data not appears in User Insight Search or Feature Flag drop down

Here is my understanding

For a specific custom data to appear either in User Insight Search or Feature Flag drop down we first need to send it via User Attributes
I updated a few users who are already present in the user insight to have a new custom data. I did it via a POST to the endpoint https://api.taplytics.com/v2/users?
I also see the new custom data when I get the user info via GET to the same endpoint https://api.taplytics.com/v2/users?

However I do not see the custom data appear as a filter in User Insight Search or Feature Flag drop down.

Is there anything I am missing ?