ADD missing in detail grid in INLINE mode

I have a Master/Detail Grid as below setting:

// enable CRUD for master grid
$dg->enable_edit('INLINE', 'CRUD');

// enable CRUD for detail grid 
$sdg->enable_edit("INLINE", "CRUD");

Why the icon ADD doesn't show on the Detail grid?

Due to the functions lifecycle, the enable edit function should be called AFTER set_masterdetail.
$dg->set_masterdetail($sdg, 'orderNumber');

$sdg->enable_edit("INLINE", "CRUD");

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