Adding to the Workers theory: if it is a Worker, Cloudflare adds a CF-Worker request header to every subrequest a Worker makes, and its value is the zone the Worker belongs to (e.g. something.workers.dev or the owner’s own domain). It’s not something the script can strip, so if you add that header to your reverse proxy’s log format you should be able to see exactly whose Worker is probing you.

That gives you two practical options: report it through Cloudflare’s abuse form with the zone name (they do act on Workers being used for scanning), and/or drop any request that carries a CF-Worker header at the proxy, since nothing legitimate should be hitting a DNS-only Lemmy host through a Worker anyway. Federation traffic from other instances won’t have it.