M2.3 - Customer has a product in their basket that is out of stock Now unable to add any product to cart

Question

Magento 2.3.3

If the customer has a product in their cart that is out of stock (gone out of stock since they added it to their cart) they can not add other products to their cart. When they try to add other products to the cart they just get the error that says the (other) product is out of stock and nothing is added to the cart.

Example:

So the customer already has "Product 1" in their cart which is now gone out of stock, then they try to add "Product 2" to their cart. They get this:

We don't have as many "Product 1" : "Size" - "6" as you requested (available qty: "0").

Product 2 is not added to the cart.

Any one seen this before?