Hello. Thank you for contacting the Toolset support.
Do you mean that whatever Toolset block I add within the view's loop output and add the custom class from advanced setting it does not get displayed on frontend?
Can you please add a space once you add your class name. So, for instance, you added the class name as customclass and then add a space and then you should save it.
As soon as you add a space, the class you added will look like as you can see with the screenshot:
=> hidden link
After that, you should hit the update/save button for your archive. Can you please confirm it works?