It's happening because you have the Inventory Check on cart load disabled.
This is a new option that exists to improve the checkout performance, but you can easily disable it on:
Admin > Stores > Configuration > General > Catalog > Inventory > Stock Options
Disabling the inventory check can improve performance for checkout steps, especially when there are many items in the cart. But if this inventory check is skipped, some out-of-stock scenarios could throw other types of errors, including:
- The requested qty is not available
- Unable to place an order: Enter a valid payment method and try again.
- Unable to place an order: There are no source items with the in-stock status.
- The shipping method is missing. Select the shipping method and try again.