Dear @Bruno_Suraski

not sure, but there might be some geometric constraint, that is missing.

my screenshot shows a

_box

_offsetSrf

_planarSrf

_intersect (Planar Srf and offset resulting in red curve)

_pipe arround red curve (blue)

_booleanDifference

some fancy and maybe unexpected geometric things are happening at the edge.

the cut, that is done by the pipe (or by a sphere rolling around…) this valley-like cut as a equal depth.

but the equal depth is not resulting in equal with of the valley…

even this is not a solution - maybe it helps to understand the problem

best

tom

(edit)

the phenomena is still there, if the corner is not part of the valley…