Jeff, I see that the one surface went missing whenever you Union welded the larger vertical block to the smaller one.

One way I found around that was to 1) use the top of the smaller block to project it's shape to the top of the larger one.


Then 2) Extrude that profile from the face of the larger shape to where the top of the smaller one used to be.


Then 3) Union weld the top extrusion to the bottom block.

It seems that the two blocks had a lot of space sharing issues, and a fight ensued. ...hope this works the same for you.