Monzo has gone from being one of the best at knowing how to handle name changes and being respectful to being really bad. This is even worse than my recent experience. Reason is none of their business and I would strongly encourage you to open a formal complaint.
My recent experience: Monzo now wants deed polls to be handwritten or stamped by a solicitor
I did just open a formal complaint, and your experience is even worse so definitely do do this as soon as you can. Maybe if they start getting formal complaints, whoever at Monzo decided to throw away their previous (correct and really respectful) policies will be corrected and they’ll fix this.