Interesting. According to the Facebook Crawler link provided by the OP [1], it makes a range request of compressed resources. I wonder if the response from the OP’s PHP/SQLite app isn’t triggering a crawler bug.

Maybe Cloudflare can step up and act as an intermediary to isolate the problem. Isolating DoS attacks is one of their comparative advantages.

[1] https://developers.facebook.com/docs/sharing/webmasters/craw...