Is there a large number of fields and content on this page in the admin? If so then you’re likely seeing a timout issue. Unfortunately there isn’t any fix for this. The problem is summed up very well by another recent topic https://support.advancedcustomfields.com/forums/topic/better-performance-when-saving-large-numbers-of-fields/
But there is a work around. It goes like this
1) Save post
2) Wait for the timeout
3) Click the back button in your browser
4) DO NOT CLICK THE UPDATE POST / PUBLISH BUTTON sorry for yelling, but this is very important
5) Click on the reload button in your browser, keep doing this until you see all of the changes you made.
Why does this work? Because of the way PHP works, it keeps on processing the submitted data even after a time out. PHP does not know that your browser timed out until it finishes and tries to send you information. It won’t try to send you information until it’s finished with what it’s doing.
