With an explicit complement, qabl is ordinarily declinable; with a specifically intended but omitted complement it can become indeclinable on ḍamma. Similar-looking expressions thus receive different parses.
Additional example: جِئْتُ مِنْ قَبْلِ الْفَجْرِ.
I came before dawn.
Min: preposition; qabli: governed noun and muḍāf; al-fajri: genitive complement.
Additional example: جِئْتُ مِنْ قَبْلُ.
I came earlier (than the contextually understood point).
Qablu: built on ḍamma; syntactically genitive after min.