That’s how roaming works. It is a temporary or backup solution.

Your data and its traffic needs to be routed back to your home country’s network, so it’s like you’re connected to the slowest vpn

Also, if networks detect roaming for more than a few months, the local network can ban your phone from roaming

i said sometimes.

my phone was blocked by local provider 4 months after roaming in uk

and that was just backup phone, barely used