I think you might've fixed it, thanks!

For anyone else reading: the issue happened on iOS Safari. I was signed in on the YouTube iOS app, and I was signed in on Google.com on Safari, so I assumed there wasn't anywhere else I needed to be signed in. But sure enough, I went to youtube.com in Safari and wasn't signed in. Tried it once after doing that and it worked. Will see if it solves it, seems likely.