In a nutshell:
What @Teo wrote about IPv6 to IPv4 via NAT Gateway was spot-on
Consequently, the packets that arrived at the server were somewhat garbled around. Possibly split up irregularly due to IPv6 to IPv4 translation. I’m no protocol expert so dunno for sure really.
What I know is that my server didn’t like that and dropped them silently. Which, to be clear, should not have happened. So the real fault was that I configured the nginx to be too strict.