Not a bug, per se, but a problem with naming. Since the sales quote Heading field can be renamed, there is potentially a built-in discrepancy between the Heading and the reference number field name. The sales quote form names the reference number as Quote number. And so it appears, regardless of whether the sales quote itself is titled as a Pro Forma Invoice, Proposal, or anything else.
I recommend the field name be changed to Reference, as it is on several other forms. This is how it was formerly.