Some field values are not saved

I use the undocumented Tags and Featured Image fields in my setup. I notice that the field values are not saved on a post update or publish. The fields are empty after an update or a publish event.
dshanske/indieweb-post-kinds (GitHub)

adds support for responding to and interacting with other sites using the standards developed by the Indieweb Community – dshanske/indieweb-post-kinds