Custom fields flow easily from customer to invoices etc. - which is ‘header’ level information. i.e. Invoice is for a customer. Custom fields from items flow to invoice lines - 'line level; information. This works great!
I cannot however get other line level fields to flow to invoice even when checked to include on documents - specifically custom fields on billable time records. These records are the source of invoice lines. I may be missing something, but I would like to find a way to flow these line level fields to the customer invoice in the same manner as already works for items.
There is currently no way to do that. When choosing an inventory or non-inventory item, the custom field comes across with the item. But when creating invoices from billable time, you are not incorporating an item, just a general ledger transaction. So you would have to create a line-item custom field for sales invoices and enter content manually.
This has been implemented in the latest version (22.8.12).
If you want billable time and sales invoice lines to share the same custom field, create new custom field where you select two placements as per screenshot below: