Well, since we're talking about behavior based on a fix that isn't written yet, things are a bit theoretical. 🙂
Here's one approach we'd consider. An expired key pair cannot be used to sign a JWT, so we would either have to generate a new key pair ahead of the expiration, or start failing login operations. The former is a better user experience, so a user will either have to regenerate the key, or we would do it based upon a configured policy.
Also, wanted to be clear that we are aware of this limitation, which is why we set the default expiration period to 10 years (so we have a bit of time to solve this in the best way possible).
Hope this helps. Let me know if you don't have the information you need.