I had a similar problem with a two form, one service solution where different users are filling out each form. The second survey pulls down existing surveys through the inbox for editing. The second survey is supposed to add the username and email metadata but it doesn't add those values. This was replicated with a case I had with ESRI "Confirmed that when the second survey is used to create a new submission, it populates username and email without any issues, but when it is used to edit an existing submission from the inbox, it does not write into those fields anymore". The analyst assigned it to the same BUG-000113164. My workaround was to use a pulldata function to add the email addresses.