/fill replacing leaves with any block is broke

I planted a lot of trees in my map, and I wanted to replace all leaves IDs with only one block, just to prevent them from decay. I tried to transform all the different leaves data in only one block, but failed - it only works with some blocks. See the screenshots for understand better.

1- The entire cube is made of leaves with data value of 1 (spruce, check_decay: 0, decayable: 1).
2- The command run must replace all the cube with stone.
3- Instead, it only replaces some blocks in a very strange pattern.
4- See what happened.