This is more or less related/similar to this bug which was fixed: (in 2003!!) https://bugs.gnucash.org/show_bug.cgi?id=103345 When you duplicate an invoice (which is effectively a "new invoice") it should allow to "edit the note" in "Edit Invoice" dialogue. Currently it is made read only. Steps to reproduce: 1. Select any existing invoice and duplicate it. 2. Note field is "read only" in "Edit Invoice" dialogue that appears. PS: I can edit the note afterwards though, in "Edit Invoice" tab page that appears after the invoice is duplicated.
Thanks for your report. When invoice duplication was introduced this read-only effect was not considered. I have pushed a fix which will appear in gnucash 3.3.