As I've previously said on here disabling IPv6 should never be an option. For troubleshooting, sure, but it isn't a fix and shouldn't ever be suggested as one.
Instead, I think logging a fault is a good start - IPv6 is production grade still and should perform the same as IPv4 (noting also it is enabled by default with all customers). Digging into it deeper with "Stats for nerds" to determine what YouTube cache you're connecting to could help too in order to see if this is a common trend with everyone here.



