You are correct. Seems to be only Automattic properties.
Thank you for looking into this. I think I found a way which hopefully avoids the infinite loop. This code refers to the Meta Title and OG/Twitter Title but I use same approach for Description fields:
https://gist.github.com/donohoe/a6cfcf5e920e4e3ee29f2bedc1847a53
There is still a separate problem on the frontend. While the PHP code appears to work, the placeholder attribute on the field INPUT and TEXTAREA elements still reflects the original behaviors.
I am solving for that with this JS though I don’t like using setInterval this way or hard-coding ID names. I may modify it to only run if the plugin HTML is in-view
https://gist.github.com/donohoe/393a795b764a4adefeaa5b8960c23908
