Here is what actually fixed it: https://github.com/yt-dlp/ejs/pull/53/changes
yt-dlp is relatively stable, but still occasionally breaks for long periods. I get the sense YouTube is becoming increasingly adversarial to yt-dlp as well.
I don't know the details, but it doesn't seem like yt-dlp is running the entire YouTube JS+DOM environment. Something like a real headless browser seems like it would break less often, but be much heavier weight. And Youtube might have all sorts of other mitigations against this approach.
I'm pretty sure yt-dlp is filled with these kinds of gold.
IIRC they maintain a minimal execution environment that is able to run just the JS needed to pass a few checks but this breaks too often enough that they're planning to make Node.js or another JS interpreter a hard requirement (possibly already happened).
I rarely use yt-dlp anymore.
Before I just updated. Now when I do that, it usually becomes complex and full of questions.