Having an old issue which was resolved reappear.
If you have a large tile render with say 100 tiles which only has information in the alpha, the assembled image will be a greyscale only image with no alpha. This bug got cfixed, the work-around is just to put a small object in the the top left corner so the Assembler realises there is information it would appear.
In Deadline 6.0
We have a similar problem,we uses the same solution as you, a small object in the top-left corner…
http://forums.thinkboxsoftware.com/viewtopic.php?f=11&t=9439
This problem was in 5.0 fixed in 5.1, back in 6.0
Looking forward to Draft replacing the Tile Assembler?
Hey,
I am definitely going to sit down with the devs and see if this is fixed in 6.1. Thanks for letting us know!
PLEASE can I have compression with TGA files whilst you’re looking at it! 
PLEASE!!!
Oh and if you hadn’t guessed from my last post this is happening with TGA files.
The new Draft Tile Assembler supports assembling TGA files which are compressed TGA’s or un-compressed TGA’s. The final TGA image Draft creates is un-compressed. Are you referring to compressed TGA’s not being supported for the tile assembly (image read) or the final assembled TGA image (image write) not having the ability to be compressed?
The Final Assembled TGA file not having the ability to be compressed is one of our bug-bears… I know it’s a problem with Draft as well and have requested it there several times too.
Hello Dave,
I just checked and that is on the wishlist for a future build of Draft, I just don’t know when that might be coming at all. I know it isn’t much, but I can at least confirm that this is on the wishlist.