Not sure that sanding the edge flat was a sensible thing to do. LR design the parts deliberately. In this case, if you look at the first photo where you have cleaned it up it shows a consistent contact point around the inner edge. The rest of the edge where the bolt holes are positioned are slightly curved down; this is probably deliberate. Think about it fitted. It has the gasket in between and as its all tightened up the bolts have a clamping effect to achieve a consistent pressure seal for the gasket. By flattening it you are reducing the clamping ability of the bolts to achieve a seal. I'd check this point with a supplier to see if new/ replacement sumps are the same.