Thanks @arkadi and @Aleks, I have tried your solutions but the problem persists.
Don’t worry about it, thought — I had to have this server running by Sunday so I installed it somewhere else, and even if I still get that the “resolv.conf file has been manually edited” —which is not true—, I am able to run diagnostics and everything seems to be fine.
I have installed YunoHost 11 on top of Bullseye and basically the findings are the same as the ones described here.