Yes, spritesheet gets jumbled because it gets optimized and you have redundant sprites. Which means you need to make sure each of 8x8 tile is unique to make it look same. Although I would happy welcome ability to turn off this optimization as it is difficult to have each 8x8 unique when you have bigger sprites.
I would suggest looking into the Sprite Builder tool. Due to the nature of how PV8 works, sprites will always be locked in at 8x8 during the import process. I may allow different size sprites in future versions, fro example the next supported a special mode with 16x8 sprites but the underpinnings were still based on 8x8 sprites in memory.
The Sprite Builder will allow you an easier way to rebuild the optimized sprites into larger ones for more complex games.