I have confirmed with 100% certainty that the post_title, post_content, and post_excerpt fields are all completely empty. I have also run a full plugin conflict test.
This proves something is programmatically filtering one of the core post data fields with an array before it's saved.
I've pin pointed it to GROUPS. Adding field in GROUPS there is a fatal error that breaks saving functionality.
I don't have time to do debugging for you on your product right now. Maybe in the future. Just know this is tied to groups, maybe your beta version fixes it if you completely rewrote that system.