Visit other .internal site -> uses TLS cert NOT signed by root CA that is preloaded on your device -> certificate error, and cannot be bypassed due to HSTS.
edit: ah, unfortunately it's not really standard, just a grassroots effort https://ungleich.ch/u/projects/ipv6ula/
Non routability was a design feature.
I've been out of Enterprise IT for 15 years - but if I was going to do an IPv6 deployment today - I would strongly consider NAT6 prefix replacement - it offers 90% of the benefits of native IPv6 addresses, doesn't conflate "security" and "flexibility" (prefix replacement is just a straight 1:1 passthrough - globally routable) - and who want to go update all their router configs and DNS every time they change their upstream. Ugh.