This kind of problem is usually due to a conflict in consequence of the rules applied when the ticket is created.
Consider that, if both Assignee and Group keywords are present in the body of the email, the Assignee must belong to the Group.
If a Category keyword (it is the Request/Incident Area) is defined in the body of the email, it could have associated an Assignee and/or a Group and, if the ‘Area_Defaults’ option is installed, the request Assignee and Group are populated with the Request Area defaults.
There could be different scenarios; for example, if Request Area has the Assignee associated and the Area_Defaults option is installed, if in the body of the email this request area (in Category keyword) and Assignee and Group are set, if the Assignee defined in Request Area detail doesn’t belong to the Group defined in the body of the email, the result is a ticket with Assignee <empty>.
That because of different and conflicting information provided: the Assignee cannot have at the same time the value provided by the email and the value set as default in the Request Area (Category) and, being the Assignee set in the Request Area not belonging to the Group coming from the email, it cannot be set in the ticket together with the Group; the ticket has so only the Group set while the Assignee has no value.
If the Assignee set as default in the Request Area should belong to the Group coming from the email, the final result would be a ticket with the Group coming from the email and the Assignee coming from the Request Area default settings.