Looks like multi select needs the (+) icon on hover moved up and to the right to match the single select module:

Here’s my temp solution…
// temp fix for ACF (+) button on multi select
add_action('admin_head', 'squarecandy_acf_multiselect_fix');
function squarecandy_acf_multiselect_fix() {
echo '<style>
.acf-taxonomy-field[data-ftype="multi_select"] .acf-actions {
padding: 0;
margin: -9px;
}
</style>';
}