Why does the middle ring on my Whirlpool burner stay cool?

If you want certainty, you can pull the element and check each section with a multimeter for continuity. No reading in the middle section means the element's bad. Otherwise I'd just live with it unless the cooking performance really bothers you. If you ever do decide to swap it, the replacement element is inexpensive and it's a pretty quick job with the glass top lifted.

New here so not sure if this helps, but my old glass top had one burner that did something similar and it turned out to be the switch going soft, not the element itself. The coils looked fine but it just wasn't sending full power anymore. Swapped the switch and it heated evenly again. Might be worth considering if the multimeter shows the element is okay.

Before you pull anything, check the wiring under the burner. On mine, the middle lagged for months and it turned out one of the wire connectors had cooked itself loose from all the heat cycling. Snugged it back on and it's been fine since. Power's off at the breaker first, obviously. Also, does the middle ever glow on its own when you set it to the inner ring only? If it does, that tells you the element itself can still get hot, which would point more toward a connection or switch issue than a dead section.

I fix these for a living, been in the trade about 14 years, and I'd add one more thing to check if the element and switch both test fine. There's a little limiter clipped under the center of most of these dual elements, and when it starts failing it cuts power back partway through the heat cycle. I've had a couple come across my bench where the burn acted exactly like yours, edges hot and center lagging, and the limiter was the culprit. They're cheap and usally just unclip. If the element and switch check out, that's my next stop before replacing anything else.

Update: finally had a free evening so I killed the breaker and pulled the burner out. Both wire connectors looked solid, no scorching or looseness. Ran a meter across the element and every section rings out fine, wich has me stumped. The repair guy mentioned a limiter clipped under the center. Is that thing replaceable on its own, or do I need the whole element if it's the culprit?