max-232's post in Disabling IPv6 is not persistant was marked as the answer
Good to know, let's ignore that part.
Browsing to the topics I did find one other topic: impossible-to-disable-ipv6
This talks about the same issue but for a secondary ethernet port.
With nmtui-edit -> edit -> IPv6 Configuration, set 'Automatic' to 'Ignore' I did finally disable IPv6 permanently.
This solves my problem but I'm still wondering what the IPv6 setting in armbian-config is supposed to do and if it is working correctly.
