A post I happened to read here on DEV linked the W3 Validator so I thought what the heck, let's see if anything is wrong, and ran my site through it. Sure enough I had a few mismatched tags. And wouldn't you know it, fixing that made the problem go away and showed me those beautiful fireworks.
Most browsers will silently fix mismatched tags, so you won't notice any errors on the site or console and it will even be corrected in the source code. But I suppose this makes the renderer waste some time re-parsing the DOM -- not enough for me to notice on my notebook, but substantial on Lighthouse's simulated slow device.