> When you use a hotspot on iOS, your device treats your hotspot's network packets differently than the network data that apps use. Hotspot packets get routed differently, and they get metered

The OP doesn't mention it but it looks like a case of where the operator monitors TTL of packets to detect tethering.

E.g.

https://piped.video/watch?v=alMsu_tfZPE

https://github.com/RiFi2k/unlimited-tethering