chopping the corners off a block

[micheus]

Did you see that selected vertex in AfterDissolve? If so, you were looking to the "bottom-left" vertex.
Select its respective vertex in the back side (the view you have in AfterDisdolve2) and then, in the context menu, select Connect.

That strange faces created after dissolve are a visual issue, that should disappear when you close the mesh you just opened.