There are a variety of variables but I’ve seen random 525s before. It depends on whether it keeps happening, or is a temporary issue that goes away. Of course if it stays like that, you would need to take action to get a secure connection again.
As the name suggests, the issue is when Cloudflare tries to establish a secure connection to your Fly app, it fails. So the request never reaches your app, and so yes, it would not be logged.
There are a variety of approaches to fix it.
Handle TLS in your app (rather than have Fly’s proxy handle it). Perhaps you already are, but if you scroll up and down this lengthy thread, that seems one reliable way to not see a 525 again. Check out this great post:
Experiment with Cloudflare’s encryption options. See which fixes it. Full or full strict are better, however can result in this issue, as Fly tries to connect using https to your app:
You could use an unencrypted connection between Cloudflare and Fly. Again, (1) and (2) are better, but then you don’t need to be concerned about a failed SSL connection between Cloudflare and Fly, because there won’t be one e.g