Stable class with unstable property

Question

After enabling compose metrics & reports I noticed that one of my classes is marked as stable with one of its properties as unstable:

stable class Content {
  unstable val aaa: List<AAA>
  stable val someString: String
}

the class itself is marked as immutable:

@Immutable
data class Content(
    val aaa: List<AAA> = emptyList(),
    val someString: String = "",
)

which I assumed would mark all of the properties as stable o.O

As for the AAA class itself is referenced from a different grade module with enabled KMP under the commonMain source set:

data class Price(
    val p: P,
    val d: String,
    val u: String,
)

enum class P {
    F, S, T,
}

My questions are:

1. Why @immutable would not mark this whole class as stable?
2. Does KMP somehow interfere with the compose compiler?
3. Are there any options for me to help the compiler?