Ask HN: Front-end bait and switch?

First-paragraph TL;DR: A weird and very specific thing keeps happening to me: I tell a recruiter that I don't do front-end work, I take an interview with an employer, I make it clear during the interview that I don't do front-end work, I hear nothing but "Yes", I take the job...and then I get assigned front-end work. My performance drops off a cliff, I try to communicate, it gets worse, and if I don't quit, I get fired. Once could be a fluke, but it has happened several times. I don't understand and would seriously like to understand so I can avoid it.

Here's the most recent example: I interview in December, it's great, there's a former colleague that remembers a lecture I gave, he's a cool guy, and I make it clear to new boss that I don't do front-end work, but to be sure, I tell them during the last round of interviews not just that I don't do front-end but that I keep getting this bizarre bait and switch. New boss nods, talks about scaling problems, they need someone that's good at Postgres. I sign a job offer and my first week, I get "please move these buttons". I protest, he says it's to get me used to their workflow. So I move the buttons, then go back and forth with QA for a week on spacing issues. I get a bad feeling and remind new boss. Similar weeks go by, I get invited to a meeting, HR CC'd: I'm slow and my work is bad. I tell them I'll do whatever they want to pay me to do, but I do a better job if I'm doing the work I am great at, "I mentioned this during the interview, I would not have taken the job if it was front-end". New boss stares down and does not say anything for while the lead talks about everyone on his team being a full-stack developer so I need to acquire front-end expertise in a hurry. I spend my evenings and weekends learning Vue.js, doesn't go well, another meeting where new boss won't make eye contact and now he's old boss.

I still don't understand why. I've started getting somewhat superstitious: I'm leaving JS/CSS off my resume and LinkedIn, I tell recruiters that I don't know JS. I hear about the difficulty scaling their public-facing APIs, the new customer that requires rewriting the ETL pipeline, I sign up, but I arrive and hear "We'll do those things later, first we need you to make a landing page and help port the front end to the new version of React."

It's gotten maddening: at a previous employer, I started having panic attacks and chain-smoking (both fixed now). I design and build APIs, I write back-end code, I optimize Postgres/MySQL/Redis, I build telemetry and then use it to implement data-driven rate limits to mitigate DDoS attacks, I architect systems, I transform and move massive amounts of data quickly and reliably. I'm very good at those things just not at browser front-ends.

I can't rationalize it as coincidence or a fluke: at least half a dozen employers have told me I'd be doing back-ends and ask me for front-end web development. I've worked at a lot of startups and have done plenty of consulting jobs: I understand wearing several hats (it is one of my favorite things about startups), and I like jumping in to help where it's needed, even on the front-end.

Has this happened to anyone else? No one I have worked with before seems to have this problem. Why would an employer do this? That's important to know: if I don't know why, I don't know what to do to prevent it. Since I am well into the senior/staff level and skilled, I cost way more than a junior front-end dev, so it can't be for economic reasons. It has happened too many times across too many years to be a weird employer or a coincidence. I don't want to come off defensive or combative during an interview, but even being extremely direct has gotten the same result, so it can't be ambiguity. Is there some magic phrase I can use with employers to say "Seriously, we will both be unhappy if you want me to make a web front-end, this will be a catastrophe"? What am I doing wrong?