One of the users has access to a Jira Product Discovery Project with Roles ( Administrator, Creator and Contributor ). Technically she is the administrator of this JPD project.
She does not have access to any other Atlassian Product.
The problem - In the comments section of an Idea, she is not able to Tag a user who could be someone like her who just has Jira Product Discovery Access or someone who has access to both Jira & Jira Product Discovery access. Technically she is not able to browse users I guess.
Since JPD is a Team-managed Project, I don't think it has anything to do with permissions schemes etc.