If both the WP-Blocks an WP-Table reloaded are installed then using visual editor causes a problem.
What happens is that when you update the page/post, then it looks like wordpress loses the additions.
In some instances, there are two versions of the page saved, and the older one does keep the newer changes - but it looks like that is only if the page has been auto saved.
Disabling the visual editor solves it, but that's not good for most users.
Uninstalling either plugin also resolves it.
I hope that saves someone the three hours I spent in trying to locate the issue!