You were right of course... I filled the missing IPv6 in and then quickly decided to change the config towards "pihole=local dns server" as I´d like to see the clients discerned. I will further observe what happens... but I am pretty confident that configuring IPv4 AND IPv6 should do the trick now 
This post of yours helped me as well: Pihole blockt nur teilweise, IPv6 das Problem? - #2 by Bucking_Horn
Many Thanks, much appreciated!
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