Recently I've noticed during at least one torrent that a certain peer seems to be suddenly recognized simply via "self connect" as if it were myself, while I was able to download from that same peer just fine a few hours ago.

It's clearly impossible that this error message is true, as this peer is identified via IP to be from america while I'm in europe and not using any VPN, proxy or thelike, so the algorithm for detecting this seems to be faulty.

Tixati uses the PeerID to detect self-connect, which is the ONLY reliable way to do it.  The remote client you are connecting to is reflecting your PeerID back to you, which means it's not acting normally and is probably a bot or reflector or some other parasite.  Nothing to get worried about.

Well that is good to know as I never heard of such a bot activity before, but remains rather annoying when that same IP was the only proper seeder for that torrent.

Another thing to examine is the port being used.  Is it the port you have set for yourself? I have a dual WAN router I am experimenting with so I see self-connects coming from 2 different IPs but both the same port.