The commenatries do mention that it can also be mu'rab when the mudaf ilayhi is unmentioned but intended, the distinction been made between niyyat-e-lafz and niyyat-e-ma'na, the mu'rab state being the former.
Qabl/ba`d when their mudaf-ilay is explicitly stated/madhkur then both are mu`rab. When mudaf-ilay is ghayr-madhkur then regardless of an `aamil preceding them, both will remain Mabni.
Ibn Malik رحمه الله prescribes the following in his Alfiyyah (410-11), (واضمم بناءً غيرا ان عدمت ما ) ( له أضيف ناويا ما عدم) (قبل كغير بعد حسب اول ) (ودون والجهات ايضا وعل) 'give damma in state of binaa to ghayr if absent is what, it was mudaf to.....' 'qablu is like ghayru, ba'du....' I.e., when the mudaf ilayhi of qabl, ba'd etc. is muqaddar it is indeclinable, mabni on damma.
[Surat 'Āli `Imrān] [Surat Al-Ĥadīd] Why is it in the first Ayah <<مِنْ قَبْلُ>> and in the second <<مِنْ قَبْلِ>>? In both cases it is majrur. The same question also applies to <<بَعْد>>. In most cases it is <<مِنْ بَعْدِ>>, but in the second Ayah it is <<مِنْ بَعْدُ>>.