have a look at ‘debug’ (under the FORM menu item) where you can preview the form but also see what actions/events are running as the report runs. Could there be other actions on the form that are influencing what you are trying to see here.
Also.. see if adding in a new page to the current report, and then setting up the same filter input object also presents this situation. if it is all OK there, then delete the object you are having the problems with and add in again.
if that doesn’t help, can you contact us directly at firstname.lastname@example.org with your account details and we can look to review the server action logs to see if that helps identify what is going on here