Submit button on form greyed out


Simple problem - I have a form with only a single required field, which is an attachment, and a few optional fields, however the submit button remains greyed out when the required attachment has been added, and only becomes available when one of the optional fields has something entered into it.

I have tried fiddling with everything I could think of - making different fields required, then removing, making a copy of the form, etc, and nothing has fixed this. I do not want to make either of the other two fields required, only the attachment.

Screenshot of the form with an attachment, but submit button greyed out:

Once anything is put into either of the other two optional fields, the submit button becomes selectable: