Summary

• When a user tried to group issues on board by Subtasks, it does not work
• All affected users belong to Member user role.
• Administrators do see this issue
• Also, no error shoes up when this operation fails
• Other group by operations work as expected

Environment

JIRA cloud

Diagnosis

Try impersonating as the affected user so that you can verify the problem.

In this scenario, user cannot see to which Epic the issue is associated with. 

Even though user has access to all the issues types, affected users could not view Epics.

Cause

It probably caused due to "Security Level" was set to "Administrators" only on Epics. Due to this, all users having Member roles could not view them and eventually the group by subtask action was failing issues could not be segregated based on Epics.

Solution

Either remove security Level or add users with Member roles also so that they can also view the Epics. 

More details on Issue security level can be found here