Error when converting BehaviourSubjects in Angular

Let's assume we have two interfaces:

interface A {
name: string;
_id?: string;
}

interface B {
name: string;
_id: string;
}


When assigning them to each other, typescript behaves like I would expect it:

const a: A = { name: 'a' };
const b: B = { name: 'b', _id: '1' };
const c: B = a; //
const d: A = b;

const getA = (): A => {
return b;
};


In line 3, I'm getting the following error:

TS2322: Type A is not assignable to type B
Types of property _id are incompatible.
Type string | undefined is not assignable to type string
Type undefined is not assignable to type string


That make sense, because the _id of a could be undefined.

When I use A with an BehaviourSubject likt this, I'm also getting an error:

const behA = new BehaviorSubject<A>(a);
const behB = new BehaviorSubject<B>(b);

const getBehA = (): BehaviorSubject<A> => {
return behB;
}


S2322: Type BehaviorSubject<B> is not assignable to type BehaviorSubject<A>
Types of property observers are incompatible.
Type Observer<B>[] is not assignable to type Observer<A>[]
Type Observer<B> is not assignable to type Observer<A>
Type A is not assignable to type B
Types of property _id are incompatible.
Type string | undefined is not assignable to type string
Type undefined is not assignable to type string


For me it is not really clear why. If I try the same with a simple container interface, it works fine:

interface Container<T> {
value: T;
}
const getCA = (): Container<A> => {
return cB;
}


(Don't know what to write here)

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