See the docs below ModelChoiceField.iterator (Not the docs for iterator itself, but the paragraph below it.)
Briefly, the __str__ method on a class is used to generate the entries for the select list. You’re seeing the output of the default __str__ method for your class.
You can either provide your own __str__ method for that model, or you can subclass the ModelMultipleChoiceField as described in those docs. (You may need to do this if you have other representations for that model where you need to customize the __str__ method.)