The Owner pull-down from the task board view in the Modern UX does not show all resources from the project team, it only shows resources that the user has view rights.
Steps to Reproduce
This is working by design.
You should only be able to add task owners for people that you have access to view as it could be anyone in the system. We specifically implemented the Task Owner and To Do owner as a standard dynamic lookup which has the security clause incorporated into it for retrieving any resource for which the user has access to.