I recently had issues making this work on some fields because their keys were formatted a little differently, causing the substr call to get the wrong value. You may be able to solve for this by working from the end of the field ID string rather than the beginning.
substr($field['key'],-13)
Of course, this is similarly (if not more) brittle, as it still depends on the format of ACF’s field IDs being consistent. Consider printing out your field IDs to take a look at them if you’re having trouble, and adjust accordingly.
I definitely vote for this to be a first-class feature of the plugin—it’s much less confusing than just leaving the label field blank!
