If I place a block, place a block of the same category inside it, and then delete the block around it, the zebra colouring remains.
Show the screenshot
(Sorry - deleted my previous reply)
As nieznajomyh says - could you post some before and after screenshots please?
It isn't suspicious 
What about it is?
it's supposed to remain, unless it's not what I'm thinking of. Please give us a screenshot.
-_-
I found that bug when I first used Snap but I think its not a big problem.
If author of this topic isn't interested, I would leave this discussion.
I cannot seem to get the bug to work now. Sorry!