ACF uses a Priority of 10 to do it’s own work on the acf/save_post hook.
If you use a priority < 10 then your action will run before ACF. The values in the database will be the previous values and using get_field() at this point will return then previous values and not the new values that need to be saved.
If you use a priority > 10 then your action will run after ACF has saved the new values to the DB. Using get get_field() here will return the newly saved values.
The $_POST value that you should be looking at depends on what version of ACF you are using.
In ACF5 it is $_POST['acf']
In ACF4 you need to look at $_POST['fields']
The nested index of the array for each field is the field key. For example $_POST['acf']['field_1234567890123]or$_POST[‘fields’][‘field_1234567890123] These values are the same no matter what priority you use for your action.
