Why typescript create warning when I copy already narrowed object property to the other object.

interface View {
    x: string;
    y: boolean;
}

type viewKeys = keyof View;

const view: View = {
    x: 's',
    y: true,
}

const allowedKeys: viewKeys[] = ['x'];

const derivedDataString: string | null = '{ "x": true }';
const derivedData: unknown = derivedDataString ? JSON.parse(derivedDataString) : null;

if (derivedDataIsCorrect(derivedData)) {
    allowedKeys.forEach((k) => {
        // warning shown here about view[k] can't be assigned to 'never'
        view[k] = derivedData[k];
    });
}

function derivedDataIsCorrect (data: unknown): data is Partial<View> {
    if (!data) {
        return false;
    }

    const _data = data as Partial<View>;

    return typeof _data.x === 'string' && typeof _data.y === 'boolean';
}