This behavior was rare in the US market, but it is very common now that our wireless providers are entering the home internet market. T-Mobile and Verizon both use CGNAT (but do issue individual ipv6 addresses to end user devices).
More specifically, T-Mobile US only assigns IPv6 addresses to devices, and to reach the IPv4 world you have to go through a translation mechanism.
Presentations from T-Mobile at NANOG and Rocky Mountain IPv6 Taskforce:
FWIW Verizon FWA does NOT use CGNAT for IPv4.
I’ve seen Comcast business also use CGNAT.