Hi @zirkonus
I found it strange too. Magento does allow natively to perform checkout after Signed In with products being added during a guest session.
However, since it isn't behaving the way it supposed to be, I want you to do Cache Management properly by re-enabling or disabling configuration settings of "Allow Guest Checkout" and test the same scenario on the storefront again. I think you will be fine after that.
let me know if the issue still persists!