Locking the Form to be signed only by certain Signature Certificates

Hello,

I have a form that I would like to restrict singing it to certain certificates.

I tried to check the box under Signature Settings/Signing certificates/Specify the Signing Certificates.

every time i hit OK and come back to the setting  I see the check box unchecked

and then i was able to sign the document with different certificate

could you elaborate me how to solve the issue

Thanks