What's your job title been at these places? Is it "Backend Engineer"? I'd have that as your job title at the top of your resume, and insist on being hired under that title. When assigned frontend work, point to your job title, and refuse to do it.

From what you've described, I'm not hearing "I don't do frontend", I'm hearing "I only do frontend badly and under protest". That's complicated, "I don't do frontend" is simple -- but you need to back it up.