Non-manifold Edges

The mating planar surfaces are not being completely removed when doing the BooleanUnion. One fix is to use ExtractSrf for this surface after the BooleanUnion>Delete it>Explode the model and then Join everything back together to fix those edges. You’ll end up with an additional surface in this workflow too that looks like the one you just extracted and deleted… delete that as well and you should have one closed solid with no non-manifold edges.