Including Custom Invoice Fields on Details Transaction Report
You're on the right track, ALImplementation.
There are only some reports that can show custom fields in QuickBooks Desktop. That may be the reason you're unable to get that particular custom fields from an invoice.
I'm here to provide a way to run and customize the Transaction Detail report. This way, it'll display the custom field and its associated invoice. I'll guide you how.
Go to the Reports menu.
Select Custom Reports and then click Transaction Detail.
Choose Customize Report.
Set the Report Date Range in the Display tab.
In the Columns section, search other Other, then place a checkmark beside the option.
Go to the Filters tab.
Locate the Transaction Type from the Filter list and then choose Invoice.
Once done, you can click the Memorize button to save the current customization settings. That way, the reports will be available for future use. For more details, I suggest visiting this article: Memorizing Reports.
Keep me posted if you need further assistance in running reports in the program. This way, I'll be able to provide accurate steps for you.