Check your script for other places where you might be setting `person`. You can also check its current value by pressing shift+o in gameplay and typing `person` then hitting enter, to see what it's set to. If it's anything other than a Term object, you have a problem. You can check if person and partner point to the same object also by seeing what happens when you type in `partner` and hit enter. Make sure you don't have rpyc files in your project that don't also have an rpy file named the same thing.
Essentially, there's no way for Ren'Py to "mix up" the two arbitrarily, so there will be code somewhere that's setting them equal, overwriting the value for person, or some other factor.