BIND9 9.21.21 DoT Forwarding Fails with Quad9 (9.9.9.9 etc) ; Cloudflare/Google/own unbound work OK

[pgnd]

i'm doing some comparative performance checking for Bind9 DoT forwarding from my local bind 9.21.21

for config

	...
	tls vm-dot         { remote-hostname "example.com"; };
	tls quad9-dot      { remote-hostname "dns.quad9.net"; };
	tls cloudflare-dot { remote-hostname "one.one.one.one"; };
	tls google-dot     { remote-hostname "dns.google"; };
	...
	forward first;
	...

with fwd from my local Bind9 instance to my VM unbound instance

	forwarders port 853 tls vm-dot {
		10.10.10.53;
	};

all's good; dns leak test from local browsers shows my VM as IP source for the DNS queries

similarly, with Cloudflare

	forwarders port 853 tls cloudflare-dot {
		1.1.1.2;
		1.0.0.2;
		2606:4700:4700::1112;
		2606:4700:4700::1002;
	};

also works, and shows CF IPs as source.

same with Google.

but, with Quad9,

	forwarders port 853 tls quad9-dot {
		9.9.9.9;
		149.112.112.112;
		2620:fe::fe;
		2620:fe::9;
	};

seems NO forwarding; dnsleak test shows Comcast & WoodyNet IPs :-/
which is annoying.

looking for errors -- not finding them, yet.  or a published reason/bug re: Bind9 forwarding 'vs' Quad9 DoT.

still digging, but ...

	... any _known_ issues with Quad9?