I got a pair of used AirPods 2nd Gen from Mercari, and while they work great, they occasional won't pair to my phone. What's the deal?

I got the AirPods from a seller on Mercari. The AirPods themselves seem to be perfectly fine, the case appears to be the issue. They show up in the search but when I tap their name, it tries and says it can't connect. I charge the case and it works. Is this what's supposed to happen, as a way to say "hey, you need to charge me!" or is there something wrong with them?