After some further thought, I believe this is working as designed, though I agree this is weird.
When you create a checkbox and provide options, we display each option as a choice.
If you didn't want it to display true or false you could create a messages entry for that field. (In the theme admin section; more here: https://fusionauth.io/blog/2020/09/01/theme-registration-form/ ).
But I agree, I think in general you don't want to use checkboxes with multiple booleans, a select box is a better choice.