The membrane should be a vapor permeable weather/air barrier rather than a vapor barrier. If it is black, it is probably 15 lb. asphalt felt.

Sheathing is required behind certain wall cladding materials but not behind others and there must be enough of it to provide adequate lateral bracing (or some equivalent bracing system).

"Normal" is impossible to define. Tell us exactly how the exterior wall is built including interior and exterior finishes, insulation, and membranes as well as a general idea of the regional climate including wind forces. The systems should all be spelled out on the drawings and specification included in the contract.

In my area, sheathing is required around door and window openings and at corners to act as corner braces. That ends up being around 80% of most structures. On most homes, it doesn't add very much time or $$ to completely sheath the entire structure instead of just the above, but you have lowball builders who won't take even that time or money to do just that.

Sure. The exterior wall will have a stucco finish and there is a wire mesh frame already put up. Beneath the wire mesh is the black membrane (so it's asphalt then?) Beneath this is either OSB board as exterior sheathing (over ~80-90% of the exterior) or the bare studs itself (in areas where they omitted the OSB). The walls will have insulation between studs, and I'm assuming a vapor barrier and drywall on the interior. The climate is mild, it rains, but doesn't snow and doesn't get too hot.

The builder is a large national "big-box" builder and this is a 100+ unit develoment. I'm not expecting custom quality, but I want to make sure this isn't a huge defect (i.e. strength of wall compromised, big problem with thermal insulation, etc.) Thanks!

Pretty common in our area,( mojave desert so.ca.), to have shear panels in strategic areas on stucco over lath homes, and unusual to have a complete shear paneling of the exterior walls,(although I would always solid shear panel). The requirements are typically corners, mid spans, and around openings where required for lateral bracing replacing let-in bracing in the old days. Before shear panel requirements, let in bracing, (1x6), was used and lathed over the exterior wall studding, followed by a scratch coat, brown coat, top color coat stucco. Im sure your home and the rest in the tract have been approved structurally and meet the minimum codes for lateral/shear panel bracing.

Unless the "black membrane" you mention is solid, this certainly sounds like an omission to me. One crew could easily do the sheathing and another the housewrap (or whatever it is). The second crew will wrap it, finished or not. One carpenter I employed used to leave out bits and pieces of the OSB sheathing at the bottom to the bitter end. If I weren't a custom builder, it could easily have ended up built that way.

In the morning call your local building inspector and ask them if it ok.

Where I live every exterior wall is sheated then it is covered in Tyvec instead of felt.

What you are seeing is common code compliant construction in most areas.

Whether or not it has been done to your contract specs is another question.

Update: Thanks everyone for replying. I did check with the building inspector, and apparently this is an acceptable practice. The varying amounts of exterior sheathing was apparently designed this way by the architect, and it apparently passed muster with the builder's engineer.