My iPhone 7 appears "Bad§String"

[Jacky0824]

when I’m on Facebook, my screen suddenly show up "Bad§String" and that time I’m scared so I just turn off my phone.

And about few hours later, it shows up again, so I turn off my phone again.
The message takes up the whole screen, and there are "Cancel" "accept" "back"

Does anyone know what happen? Is this a new bug? Or my phone is just being hijacked?

Btw I don’t know if it could be helpful for troubleshooting , but when I received that message, my Bluetooth is on

 

[C DM]

when I’m on Facebook, my screen suddenly show up "Bad§String" and that time I’m scared so I just turn off my phone.

And about few hours later, it shows up again, so I turn off my phone again.
The message takes up the whole screen, and there are "Cancel" "accept" "back"

Does anyone know what happen? Is this a new bug? Or my phone is just being hijacked?

Btw I don’t know if it could be helpful for troubleshooting , but when I received that message, my Bluetooth is on

Seems like a Flash SMS message that's likely spam or something like that. You can find some more about those in various places online, like at https://www.geekdashboard.com/stop-flash-messages-iphone-and-ipad/