not sure, but there might be some geometric constraint, that is missing.
my screenshot shows a
_intersect (Planar Srf and offset resulting in red curve)
_pipe arround red curve (blue)
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
the phenomena is still there, if the corner is not part of the valley…