Looks like you are using the HE DNS which will by-pass the T-Mobile DSN64 service, and since 127.0.0.1 is first, i think you will just use the T-Mobile IPv4 DNS most of the time, which does not serve the Google IPv6 DNS records.

My suggestion is that your /etc/resolv.conf look like this to best use the local T-Mobile DNS64 as well as receive the native IPv6 content

namerserver 2607:FB90:BEEF:A::1

This name server should have the lowest latency since it is on the T-Mobile network and also serves the native google / youtube IPv6 content