while Pligrim Reaper(Dark, Zombie, RK6) is on my field and Number 22 Fran Ken(Dark, Zombie, RK8) is in my extra deck, I can't use Rank-up magic Astral force to special summon Fran Ken.
Fran Ken cannot be special summoned except by xyz summon, but special summon using RUM cards is treated as Xyz summon.

explain why I can't or fix the bug please.

Yes monster summoned by RUM is treated as XYZ summon but after the monster is XYZ summoned.No 22 can only be XYZ summoned and summoning via RUM is special summon not XYZ summon so you cant use RUM Astral Force to summon No.22.