First: I am not familiar with acf_form(), but let´s go trough it.
Is this problem also present in other browsers?
IMHO it is related to a “bug” in jquery using
.val() on fields which are hidden on page load.
One workaround is to use
$('#myInput').attr('value', value); instead, but am I right with the assumption, that the JS comes from acf :D.