Immutables Library Does Not Require Collection Attributes To Be Set

Question

I recently started using the Immutables library, and I'm puzzled by the following behavior:

Given the following abstract value type

@Value.Immutable
public abstract class Foo {
    public abstract String bar();

    public abstract List<String> baz();
}

attempting to build an object like so

ImmutableFoo.builder()
    .baz(List.of("1", "2", "3"))
    .build()

fails because

some of required attributes are not set [bar]

yet

ImmutableFoo.builder()
    .bar("1, 2, 3")
    .build()

is fine, and there is no objection that baz has not been set.

Why do collection attributes receive different treatment?