Have you also tried making a subdomain and not making any requests to it yourself? So no browser access or other DNS resolution requests for the new subdomain. That should rule out some of the other possible causes suggested in the other comments.
How is it being logged publicly? Like OP said there is no specific subdomain registered in the DNS records (instead using a wildcard). Same for the SSL cert. Only things I can think of is the browser leaking the subdomains (through google or Microsoft) or the DNS queries themselves being logged and leaked. (Possibly by the ISP inspecting the traffic or logging and leaking on their own DNS servers?). I would hardly call either of those public.
Hmm, sounds interesting. Any interesting/notable differences that set this project apart from other similar projects like IPFS?
Locally hosted git server (fork of Gitea)