Currently ProForma User Fields in JSM do not behave the same as Jira User Fields. Specifically, customers cannot look up other customers even if they have the Browse Users permission. This is a documented deficiency in the ProForma documentation.

Now that ProForma is in-house, it would be great for Atlassian to resolve this and allow the ProForma User Field to behave the same as a Jira User Field.

https://docs.thinktilt.com/proforma/ProForma-User-Lookup-Fields.1571291698.html