Are the other networks you’ve tried from the same or different?
I’d start with traceroute and see how far your IPv6 traffic gets before it fails. It could very well be some peering or routing issue between some of the ISPs in between you and wherever that IPv6 address lives. If this ends up identifying where the traffic dies, a lot of the tier 1 ISPs have BGP looking glass servers so you can get an idea of what they know about that subnet.