How do you mean? I've only been playing as Muslims lately so I haven't had any problems with inheritance.
Basically the game no longer accepts that a character can be the heir of two different people simultaneously. So for example I was playing a female duchess. I had 5 kids, by different fathers (because of course) but were all legitimate children. My current husband is the emperor, and his son with me is his eldest son, but is my youngest son. (Matrilineal marriage, so all my children are of my dynasty. I married him before he was emperor).
The empire has Agnatic Primogeniture. Thus, my youngest son is the heir to the emperor title. So I switched to Ultimogeniture, which should make my youngest son the heir to my duchy. But it didn't - it made my 2nd youngest son my heir. And then, later, my youngest daughter became my heir, because my 2nd youngest son became the heir to some grandfather's county in Greece. But then some other branch of that grandfather's line had another child that took priority, and my son became my heir again.
(I also have a third son who I sent off as a mercenary, and due to an unrelated/possibly related? bug, he is barred from inheriting anything, even under gavelkind.)
(So yeah, this mostly would not affect Muslims, because the situation is more prevalent when two people who BOTH hold titles are married, since it creates far more situations where someone should be the heir of two people or more.)