This is reproducible in case XSLT is not uploaded. Please make sure you always upload XSLT file for data import connection you are using for XML email processing.
We provide some sample xml email and XSLT files for XML email feature in Help:
Configure > Setting Up Email > Configuring Email Servers > Creating an Inbox > Exporting Email Examples
After processing an xml email, email service will create 3 information parts in the generated xml file: the main BO (Incident or other type), an Employee (newly created or existing) and a Journal Email.
With the correct XSLT file, Integration service will create the main BO, the employee (if not exists), the journal, and link both employee and journal to the main BO, same way as processing the regular incoming email.
Without XSLT, the processing will only partially work."
Therefore the solution is creating a XSLT file and upload it in "Extend >> Integration Tools >> Data Import Connections >> Settings >> Email XML with XSLT