"I would go into this assuming that either there is something else on the page with that path (or an errant javascript request)."

There is no JavaScript present. I'm 100% certain this behavior is intentional. I've responded to the unexpected behavior (which originally caused my application to crash under those circumstances) by yielding resource generators in the specified order which ultimately upgraded the connection to HTTP/3. I can repeat that behavior, log the same path, and get a new result. I don't know what else to say about it.

"But, again, HTTP/3 does not chain together requests as a single path in the way that you described."

I wonder if that path might be how the HTTP3 request gets translated to the WSGI app. Basically... something between the Cloudflare tunnel and my WSGI app.

"Also, I'm writing a HTTP server at the moment, so I'm pretty familiar with the protocols."

Very fun! This is my next task actually. I have the TCP/IP series arriving next week and already have Unix Network Programming on the shelf.