From JSM 4.18.2 and 4.19.0 Projects which have the Customer Permission ‘Who can raise requests?' set to ‘Customers who are added to the project’ will no longer be able to add non-customer users as ‘Request participants’ during issue creation. This is due to a changes to the Customer Permission requirement.
Workarounds
Auto Join Groups
JEMH can be used to automatically Join users to a group that has a customer role on the specified project on issue creation. To configure this, navigate to:
JEMH > Profile > Project Mapping > Issue > Groups
From here add a specified group for the Project with the ‘Customer’ role and set the ‘Auto Join Groups Audience’ to All Email participants
