When you are editing the guarantee, the guarantor displays the borrower number, which is useless to most staff. The label says "Patron #" which most staff think of as their card number.
Hi Christopher, I think this has been fixed now. There was a follow-up in Bug 23808 that directly relates to the problem you describe in your report. It has been backported to the 19.11.x series on 2020-06-16 and should be available in 19.11.07 onwards.
*** This bug has been marked as a duplicate of bug 23808 ***