If you are in the Plum application and look at the Team tab for any of your ATS/HCM mirrored jobs, you will see a note explaining that the team members in Plum for that job may not be the same team members for that job in your ATS/HCM.
Team members for a job in your ATS/HCM are not automatically synced over to Plum, except for the job creator. Other team members need to be invited to the job within Plum. If you add a team member in Plum, they will not be added to the job in your ATS/HCM.
In Plum, team members of an ATS/HCM mirrored job are more than likely added as Expert Contributors so that they can create a Match Criteria for that job.