Whenever you try to change quantity from the cart and that quantity is not available in the inventory, unit price is wrong.
Example: I add Product A in the cart (simple product), first time I choose a quantity 1 from product detail page.
After that, I decide to change cart quantity to 3 and press Update button in the cart. In my inventory there are only 2 stock units of Product A.
Magento throw correctly a notice that is not possible to add 3 stock units in the cart for Product A but if you see under the unit price column, its price has been changed.
With a quantity of 2, unit price is 41,50 Euro, with a quantity of 3 unit price become 18,59 Euro. If I change quantity to 4, unit price become 9,37 Euro. So every time quantity is changed, it seems that unit price has been recalculated for some reason.
Is there a logical explanation to this strange behaviour?
Magento installed version: 220.127.116.11