This is a very specific bug...

I've made a column of dispensers and I put a bucket full of water in each of them.

When I activate all of them, the water is dispensed correctly. But when I activate them a second time to remove the water, the 6th dispenser (and only this one) don't remove the water. This bug only happends when the block below sources is a block of soul sand or a magma block.

On the screen, there is 3 columns : 

• right with a regular block (sandstone here)

• center with soul sand
• left with magma block

Sorry for my bad english, i hope that's understandable with screens.