This is not directly related to your assumption of incorrect ping usage, but maybe worth considering:
I've observed your usage of link local addresses (starting with fe80:) (click for more...)
While ping implementation in Ubuntu Bionic requires link specification (i.e. scope or interface) when pinging link-local addresses, Cosmic and Disco mark it as "can be used but it is no longer required". For complete comparison, see Ubuntu man pages for ping(8).
You might consider employing ULA addresses (from a random fd00::/8 range) instead, as recommended in Use IPv6 ULA addresses for Pi-hole.
From your post, I assume you are running a recent FritzBox (with Advanced View enabled), so open Home Network|Network|Network Settings|IPv6 Addresses and choose 'Always assign Unique Local Addresses (ULA)' from the list of available ULA options.
After renewing Pi-hole's leases (safest to achieve by a restart), you should edit /etc/pihole/setupVars.conf and /etc/pihole/local.list to reflect your new ULA-prefixed IPv6 address.
As a side note: If my assumption of you using a FritzBox is correct, then I find your gateway's link-local IPv6 address mildly confusing...
It does not seem to be a privacy extension address, yet your interface identifier does not resolve to any known vendor (where I would have expected AVM).
This might be explained perfectly by you obscuring the address when posting it, or other factors I am unaware of like more advanced setups including VPNs.
If you didn't alter your gateway's IP in any way, then you should probably double-check whether that address actually is your FritzBox.