Possible hacking of my OnePlus6. Contacts disappearing under 'Speed dial'
I doubt that your phone has been hacked. There have been other people complaining about the same problem has shown in the OnePlus Forum and even here. It must be a problem with the app itself. One simple solution will be to use another Phone app. Or if there is an update to the OS or Phone app, then update it to the latest version and check if the problem persists.