This issue has been resolved.
Some customers are seeing issues with custom messages in the native and desktop proofing viewers.
When adding a custom message to a proof, the custom message field is grayed out and does not allow you to add a custom message. If you try to populate the field, the text disappears after entering a space character.
if there is already a custom proof message from a template and you edit the field, that also clears the text from the field and reverts it to its default value.