Fresh install of WordPress 4.9.8
ACF PRO 5.7.6
Fields with the post are saving only when the input values are valid.
When you input an invalid value in the custom field and try to save the post there will occur AJAX problem (500 Server Error). You will get “Post updated!” notification (but nothing is really saved) and the updating button goes into infinite loop with “Updating…” text on it.
That breaks the whole editor, because there is no validation error at all. What’s more there is no notification like e.g. The field is required. Afterwards you can’t even save the post; unfortunately you need to refresh the page and fill all inputs once again.
Obviously everything works like a charm in a standard editor.