Genuinely, I cannot reproduce the issue. I ensured that both DoH (set to Cloudflare) and H3 are enabled but didn't hit the snag, so either Cloudflare has a buggy deployment or the hunch ( https://bugzilla.mozilla.org/show_bug.cgi?id=1749908#c6 ) that it's telemetry activating H3. Update: using Google's ( the horror ) DoH, and I didn't notice problems. Might just be CF having a bad H3 deployment, but I want to know wh…
It sounds like users with telemetry disabled were not affected.
EDIT: https://bugzilla.mozilla.org/show_bug.cgi?id=1749908#c19 this is seemingly not the case, other HTTP/3 services can also trigger the same bug, telemetry just happens to be the most common / first to be triggered