Bah, that’s weird. When you notice it go critical - running that fly checks list might get you some more clues, we put some of our check logs in that output.
Taking a stab at some things you could try though depending on your situation. How is the API accessed? I wonder if something with TLS is off and it’s manifesting as a critical healthcheck failure.
Here’s someone who only accesses their app via flycast (no public IP) that had to disable force_https: Flycast ip doesn't resolve to app. This is probably less desirable if you’re intending to access the API publicly.
You might also check if your certificates are operational: Certificate issuance taking a long time. You can check your certificates on the dashboard under [your app] > Certificates or via CLI using fly certs.