In 9.2.8 and 9.2.9 a UserMessage has one child job for all recipients; in 9.2.10 a UserMessage has one child job for each recipient
In UserMessageNotificationActionProcessor the createOneEmail method relies on the formsUsage result to determine if the user needs a special email. It appears that as soon as you have a form on your UserMessage, without the RestrictedSession privilege, the system will send individual emails.
In 9.2.8 and 9.2.9 it sends one single email after adding a form to the UserMessage.
In 9.2.10 it sends sends one per user.
No workaround is available
This issue will be fixed in a future version