Qabl means “before” and baʿd means “after.” Ask: before/after *what*? If the complement is omitted but a specific complement remains intended, the words may be built on ḍamma.
Book example: مِنْ قَبْلُ وَمِنْ بَعْدُ
Before and after.
Min is a preposition; qabl/baʿd are indeclinable on ḍamma but syntactically in the genitive position after min.