Hello, we checked in your our end and we did not find any issue. Please make sure you have enabled
i)per-product shipping module enabled by admin.
ii)Vendors have added shipping class and shipping rates for the products.
If you have done all these, then you are not supposed to get any errors.

We have enabled all these and checked by adding product from both the vendors and per product shipping was charged separately for both the products.

Here is the video
https://www.awesomescreenshot.com/video/18605761?key=2f23b27e7c7609cfba2897cfba51f246.
Kindly check and let us know.