We have a VF email that is sent via guest user to our members to check a box to authorise an Owner Change. Since Spring'21 the email is sent, we can still see the page but the checkbox is now not available to be checked and then submitted - is there any workarounds that people may have that will fix our issue without a complete change of functionality?

The object that the check box updates is 'Controlled by Parent' and Sharing Rules can't be created for the object