Autofill takes the Username entry and fills it into the first field even though when is the second field that is clearly identified in the page as Username. An example page that displays this behavior is https://licensemanager.emclient.com/account/login. It would be better for Proton Pass to fill the Username data into the Username field.
Of course I would also prefer to have the ability to also autofill the User Group field, but that's another feature that has been separately requested, but as it is, I must instead put the User Group data into the Username entry to get it into its target field, and separately copy and paste the User Name data into that field.