I can reproduce in Safari on iPhone too (using BrowserStack), and I've reduced all CSS to this to reproduce:

  @media (max-width: 26rem) {
    html { font-size: 12px; line-height: 150%; }
  }

  a { transition: all .2s; }

I added that transition for a slightly prettier colour change on hover. What was going on is that this would also animate the size of the font, triggering some back-and-forth between the regular font size, and the smaller one in the media query.

I fixed it by changing to:

  a { transition: color .2s; }

I think this is how it's supposed to work. It's just the iPhone SE just to happens to be at the exact size to trigger this? I'm not sure, I wasn't really able to trigger it in desktop Chrome or Firefox.

At any rate, thanks again :-)