When we access the project settings > People not all the users are listed there. If you filter the results by a specific role and you have a large number of users in that role(100+) not all of them are paginated.

Use case:

If you have more than 100 users in a specific role such as Jira Service Desk customers and you apply a filter to look for this specific role, not all the members will be paginated. This might be confusing for Jira admins as they will not have visibility over all the users who belong to that role.

Suggestion
Either display a warning for the admins or paginate all of the users.

Notes
If you narrow down the search by typing the user's email the user will be returned.

Workaround:
User Jira REST API to return the entire list:

Get project role for project

GET /rest/api/3/project/{projectIdOrKey}/role/{id}