Curves in Blocks never seem to be able to be in the background, they remain visible whether the drawing order is defined according to the layer or manually.
I think you are running into this issue: RH-83874 Draw order commands don’t support Blocks
When you run one of the draw order commands and try to select a block (or have a block preselected), that simply doesn’t work. Selecting the hatch and using BringToFront sends that in front of both the hatches and curves in the block: