The value of a custom field on the time tracking form is not carried over to the invoice for billable time

A custom transaction column field created in Time Track is not showing in the Invoice > Items tab > Billable Time subtab though the field is already set to show in the Invoice form. For the field and its value to be carried over to the Invoice, Combine Time Items preference must be disabled/turned off.

1. As Administrator, navigate to Setup > Accounting > Invoicing Preferences.
2. Uncheck the Combine Time Items on Invoices.
3. Click on Save.