This post is a follow-on to Is this object watertight rhino yes and Anycubic Prusa and F360 no
Explode and Join works for me in V8 with no loss of surfaces. Image below has original non-manifold object on left, fixed object on right.
Your object with the claimed lost top face has 3122 surfaces like all original object. It does have naked edges. When I exploded and joined the original non-manifold object there were no naked edges.