I have a feeling its due to how the data is inserted. Something in my head tells me that you need change the priority on the save post function when inserting data from an import.
I assume the post is created when you import from your XLS?
What does your function look like to import and add the ACF fields?
Check the docs as I think you need to amend the priority on the function. You may need to use the field key rather than the field name but think its the priority.
