QuickBooks Online has an unfortunate limitation: Only the first three created Custom Fields can be exposed to third parties, such as Vinosmith.
If the custom field is not visible to Vinosmith, it cannot be read from or written to when Vinosmith reads/writes an Invoice.
This is not a Vinosmith limitation but a QBO one, and Vinosmith has no control over this limitation.
Please see this article for more information:
https://help.developer.intuit.com/s/article/Enhanced-Custom-Fields-for-QuickBooks-Online-Advanced
Potential Workaround: Use the Department or Location list to track the Sales Rep.