Hope you are well. Can you please share the steps that you took to assign the classes to the products and we can replicate this on our end and have a look for you.
Set up new product
Select a category
Select a class
Select Inventory asset account
Select Income account
Select expense account
I didn't want to use up any more of your time in this matter.
This would be best troubleshooted if you were to reach out to us where an agent with the correct tools can help you throughout the interaction live.
Here is the link to our support page where I recommend giving us a call - https://help.quickbooks.intuit.com/en_AU/contact