This was such a helpful course that helped me understand why the pattern preview breaks and also helped me figure out that patterns (although seamless) will appear broken when layered one on top of the other if canvas sizes are different!  This is a game changer for me as I explore a lot with textures.  

This has been ongoing for a number of years and I'm not entirely sure Adobe has any more information for this ongoing issue!  

 

Cheers!