I totally missed the reference field while paying a large tax bill recently, while I was actively looking for it. My partner just did the same thing when paying another important / reference required bill.
Looking back at how either of us saw this, I tried a new payment, and finally realised how we missed it. See attached. It looks like ‘Sent from Monzo’ is a title field, exactly the same as the ‘Schedule Payment’ title. The word ‘Reference’ is then super tiny grey, like the footnote to Schedule Payment (the kind of text style that’s designed to be ignored, if were honest!). Please make the Reference title clearer and leave the field blank or something. Ta!