Floating anvil using TNT

I made a TNT canon foruse of anvils and loeaded it with 3 anvils op top off eachother , the top one ended up floating in mid air.
What I expexted to happen: either the anvil should be fired off into the air, which happend with 1,2 and4 ontop off eachother, or it should fall.
How to reproduce: Place own a pstickypiston facing up, place a slab ontop of it and then two Blocks besides it in a line in one direction and three in the oppisit and make a border around it. Put a bucket of ater in each end and fill it with 3 Blocks of TNT. The piston need to have a NOT-hate to it and 9 repeaters on full delay.
Now create 2 more besides it and place 3 anvils on top of the slabs and fire.
Using 1 on each slab will fire them in the air, the same with two. When using 4 it will still Work but the anvils dont flow that much. Only Works for anvils.
There should be screenshots showing it. I havent seen any other tickets in here for the same thing but i have looked for it in a long time.
Im placing a repeater into the indput block and then just step on a wodden pressure plate.