Unable to find incident chat participants in assign role search

Hello,
I was under the impression that in an IRM incident, when assigning roles, those users are pulled from the Slack users list, similar to how the incident chat participant list is populated via users in the dedicated Slack channel. We currently have an incident with users in the incident participant list, but those same users are not populating in the “Add user to the role” dropdown. Any ideas what might be going on here?