This chapter’s compounds have two key patterns.
11 and 13–19: both parts are fixed on fatḥ; the whole numeral has a syntactic case position.
12: the first part follows dual case endings (alif / yāʾ); the second remains fixed.
بَعْلَبَكّ: the first part is built on fatḥ; the second declines as a diptote.
Decide case behavior from the type of compound and its governing syntax, not just the look of a final vowel. The next source chapter is al-Kināyāt.