The problem that MoI hits and AD fusion doesn't, is, I think in the way the fillets are constructed. AD seems to recognise a sharp edge and when it does, it automatically extends the fillets until they cross then trims them back to a sharp edge. It must interpolate the crossing in some way, as the extended fillet surfaces won't always meet exactly. MoI doesn't have this interpolation and gets in a knot. Michael G will know why it works in one case but not another.

PS: just tried it in Rhino, which works if Distance between Rails is selected as the fillet type.