I am seeing this as well on my primary PC using 1.1.1.1 as the bootstrapAddress, and trr mode 3.

Someone reported this as a bug but then the problem went away so the bug was closed. There must be some circumstances that cause it, but they are yet to be determined.

https://bugzilla.mozilla.org/show_bug.cgi?id=1562114

I'm able to reproduce it after a clean reinstall of firefox, and deleting all local profiles. Do you know what sort of data would be useful to a bugzilla report? I'm guessing some level of network trace?

Honestly, I don't know. You could offer to help in the bug.