Powerapps - Office365Users Combox not saving on form submit

I have an app which tracks help desk tickets and allows admins to assign users to them. The trouble when trying to save changes to the details screen, the 'Assign To' combobox clears itself and isn't saved in the sharepoint list.

The app uses a sharepoint list 'Mail Tracker' as the primary datasource, and the 'Assign To' combobox is pulling names from Office365Users. Within the sharepoint list 'Assign To' is of type 'Person/Group.'

Any help or insight into the problem would be greatly appreciated, thanks!

Someone from another forum had the answer. I needed to use Choice('Mail Tracker'.'Assign To') as the sharepoint list was already able to populate the list with all the required users.