Ghost data switching on iPhone with multiple SIMs

I moved overseas bought a second Sim have both phones Sims active but data switching is turned off so the US number does not get used unless I’m attached to the Wi-Fi. While using the phone with data switching off the phone randomly switched to my US data. I double check to ensure the data switching was turned off and it was still off. This would be the third time that that’s happened. What could be the cause?

[Re-Titled by Moderator]

Original Title: Ghost data switching