Water leaks through stair blocks when not all of same type.

I built a small 3x3 farm well using 8 upward oriented stair objects.  When I fill the well with water, the water leaks out.  

I tried 'cobblestone', 'mossy cobblestone', 'brick', and 'mossy brick' patterns, and they all leaked. 

okay, I tried again and noticed that if I used the same 8 blocks it works, but if I mix and match different stair types the well leaks.

Thanks