I'm assuming you are filling this field with the default value option in the field's settings. Default value will only be overwritten when it is empty on form load so this would not be able to be set via JS.

You have two options

1. Clear the field when you are done using it in Workflow
2. Use a table instead of a single field, and the option "A data source, variable, and/or users can add rows up to the maximum." with 1 min and 1 max
   1. This way every time the form loads a new submission the new row is filled with the user that will submit it's username/email/etc
   2. Wherever you need this information just be sure to pull the last row's data for the "Last Approver"