If you are in the Plum application and look at the Hiring 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 to Plum. Hiring team members need to be invited to the job within Plum. And just like in the case of your ATS not syncing hiring team members to Plum, Plum does not sync hiring team members you've added in Plum back to your ATS.
However, this functionality exists for those customers that leverage Plum's Public API if they so choose to build it.