pipe wall loses thickness when slicing or coarsening
 

I have a problem with a pipe inside a tank. In the other end of the pipe I need dense grid. I want to have less cells, so I want to cut the pipe into three parts. Then I could coarsen these two other parts. Pipe wall has thickness of about 2 cm.

But after slicing blocks, outer wall of the pipe is merged partly to the inner wall. This happens in the middle of the pipe, both ends are fine. It doesn't make difference if I slice blocks just outside of the pipe, or all way through.

Any idea why is this happening?