EC2 instance has started refusing SSH access

I created several EC2 instances running Amazon Linux 2 a few weeks ago, all with SSH access. A few days ago, one instance stopped allowing SSH access with the message ec2-user@10.0.145.93: Permission denied (publickey,gssapi-keyex,gssapi-with-mic). - an error message I'd expect to see if my SSH keys were invalid.

The instance is still running, and the cron jobs I configured on it are still running as normal. I don't have serial console to the instance but I was able to create an AMI and then a VMDK from the instance in its current state and mount the filesystem to attempt to figure out what's wrong. As far as I can tell all the SSH configuration is still as I configured it: all the correct keys are still present in ~/.ssh/authorized_keys and the contents of /etc/ssh/ are identical to a clean AL2 install.

The security groups haven't changed (they still allow SSH access) and the SSHD config is the same, so what is going on?