This is not fixed and has been a major problem for more than a year now.  This discussion thread was started way back on July 7, 2020 and it is very easy to reproduce.  The quotes I send are NOT estimates--they are fixed-price quotes that customers can issue purchase orders for immediately.  But no matter what I put in the custom subject line field, Quickbooks adds "ESTIMATE - " to the beginning of the subject line in the actual email.  Here is an example screenshot from my email client (I have invoices set to CC me):

None of those emails included the "ESTIMATE - " part in the subject line box when I configured them. I stuck with Quickbooks online anyway because I assumed Intuit could promptly fix something like this.  Obviously I was wrong and it's making me question whether Quickbooks Online is a tenable choice for my business going forward.