I have over the last months experienced a problem with ACF that until now I was not able to understand but I have at last tracked down.
The problem I have sometimes experienced is that upon submitting to update an existing post using ACF fields the page doesn’t actually post.
The problem seems to relate to fields within a tabbed page of properties and the problem fields not being in the currently visible tab at the moment of form submission/update.
To give you a specific example that you can use to replicate the problem….
1) Setup a number field and add a min/max number range to it. Put this property within a collection of ACF fields split between multiple tabs.
2) Create/edit a post and add a number to that number field OUTSIDE of the range you allowed for.
3) CLick into another tab so that the problem field is not visible on screen
4) submit the post for saving.