That’s surprising, considering CGNAT would break it as well and is meaningfully common.

Probably, unless they have a static delegation or do prefix delegation properly, which if they did they probably don’t suck enough to require double NAT^ lol

^single NAT for IPv6, assuming they don’t NAT it themselves

You could always do double NAT (put your own router behind theirs) as last resort. It’s not that bad, I’ve done it a lot.