Calculating Shipping Costs when changing quantity

Please try the following:
Force shipping costs is disabled.
Weight is set in product.
Manual Shipping Plugin is active.
Add a product to cart and proceed to checkout.
Checkout is showing the right shipping costs.
Change the quantity of the product in the checkout window.
Shipping costs disappear completely.
Is this a bug or do I have a fault in my parameters?

It's probably because you set a weight minimum and maximum in your shipping method.
Please note that shipping methods are for the whole order, not for a product.
So if you have a product of 1kg, if you have a maximum weight of 1.5 in your shipping method, if you change the quantity of the product to 2, the weight of the cart will be 2kg and the shipping method won't be displayed.

Hi Nicolas,
thank you for your reply.

I checked this, this is not the case.
Plus: when changing back to the original quantity, the shipping costs do not appear again.

Then please do a screenshot of your shipping method configuration and give a link to your shop so that we can see what you did as it is a configuration issue on your end (it works for everyone else).

Hi Nicolas,
please find the screenshot attached.
The shop is not yet public, I will send you the url and a login via PN.
Big Thanks!

Hi Nicolas,
you can close this thread - we found the reason for this behavior.
There was a failure in the checkout workflow.
Somehow, there were two basket entries within one step (there was only one basket visible in the frontend).
After solving that, the checkout process is working fine.
Thank you!