Thanks @daringer for your reply.
Apologies for being unclear: I did open port forwarding for both, IPv4 and IPv6 initially. I edited my first post accordingly to have a clearer historical reference. (I also made it clearer that I made a DNS-Rebind Exception for myname.dedyn.io)
Following your advice, I closed port forwarding for IPv4. However, this did not change anything (even resolving the IPv4 still works, which should not be the case now?).
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Could it be that I set up the domain wrongly? When setting it up and confirming my email address with deSEC, there were some options on how wants to use the domain on the “welcome you have your domain now”-page of deSEC with the ability to check stuff, which I basically ignored, since I received the token and went on to make a password for my deSEC account. No idea whether that might be it, just stating it.
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(Also, in the Fritzbox at “Internet-Freigaben”, next to “Portfreigaben”, there is “DynDNS” which is described as " Über DynDNS können Anwendungen und Dienste, für die in der FRITZ!Box-Firewall Portfreigaben eingerichtet wurden, unter einem festen Domainnamen aus dem Internet erreicht werden, obwohl sich die öffentliche IP-Adresse der FRITZ!Box mit jeder Interneteinwahl ändert." - there, one can input DynDNS provider, domain name, account… Sounds like sth. I am trying to achieve? I know this is not mentioned at all in your guides, so it’s very likely not needed, just throwing stuff at you because I don’t know).