I just bought Astropad and I am having the same issues with pixelization.
I am running a iMac 27 Late 2013 with 24GB of memory running El Capitan (10.11.5) 3.2GHz iCore 5 with a brand new iPad Pro 12.9 256GB.
I purchase Astropad so I could do some retouching work on Photoshop.
The first time I ran Astropad it seems to work perfect. Then...
I started to run it again and whenever I make a selection with the selection tool parts of the image and parts of the program gets pixelated.
I need to know if this is normal or if there is something I can do to avoid it.