Check boxes won't stay checked

I recently created a form using InDesign 2023 and exported it as an interactive PDF.

When I open in it in Reader and I select a checkbox, a check mark appears, but when I navigate away from it (either by clicking elsewhere or hitting TAB) the check mark dissapears. And if I save the form when the box is checked and then reopen the form, the check mark is gone. What am I doing wrong?