This is almost certainly because it is detecting your SIM's country code. All apps can find out the two letter country code of the SIM that you've put in the phone.
This is known and tracked here:
https://github.com/GrapheneOS/os-issue-tracker/issues/502
To be clear, this is not a bug. It's fully documented and intended. For example, this is how an app like Signal knows your SIM's country when you're registering and changes it to that before you ever type in the number.