You are shadowing a public domain, but you have defined only A records for it.
It's no surprise that Pi-hole (6 as well as 5) returns public AAAA records as defined in public DNS.
This has been answered before, see e.g. Resolve bad canonical DNS records - #11 by Bucking_Horn
or Local IPv4 DNS records also resolving WAN IPv6 addresses - is this by design? - #4 by Bucking_Horn.