This could be user error I think …
But if I take a subD surface and use the extrude dot on the gumball to extrude the surface it effectively creates a ‘box without a lid’ (see below surface and resultant extrusion)
Why is this ? I would have expected it to just create a ‘closed box’ ?
The workaround I use is to copy and paste the source surface / extrude then select the ‘spare’ surface and join to the extruded surface… to create a closed box.