How are you collapsing the field group, is this the normal way by clicking the title area of the field group? or are you hiding the fields using conditional logic?
If it is the first, I’m not seeing this. Try deactivating other plugins and see if you can narrow down what might be causing it.
If it is the second, ACF does not validate fields that are hidden by conditional logic. Also, fields that are hidden by conditional logic are not submitted or updated.