Not a wordpress issue, please see excert from a chat below:
Assumed it was encoding issue (had just updated wordpress and database,e tc.)
Also because JSON in ajax had full content – php://input was truncated
tldr …. it wasn’t wordpress
installed dumpio and got raw apache posts
turns out php sapi was truncating the post
https://bugs.php.net/bug.php?id=69487
Although I thought I was on a higher version than that
In fact, I am – but it still exists
Somehow, big permissions bug when updating my debian version
apt-get remove –purge all of apache and php, then it started working