The shops in EU are not separate zones, they are part of the world (not an interior zone that loads). Usually MMOs use Area of Interest to accomplish this - when the player moves it calculates all of the relevant objects to load and monitor events for based on a given radius or calculated area around the player. I'm guessing when you walk into a shop, MA is setting the AoI to the bounds of the shop, probably based on the wall colliders. There appears to be a major bug there in determining the AoI, which in turn causes the scene to vanish, but as you look around or move it's recalculating the AoI and sometimes gets it right. This is all just a guess though, I have no idea how they actually coded it.
In response to the "disappearing wall/floor" bug. Does this bug refer to the fact that you can run into a mall shop really fast and sometimes the shop interior takes a couple seconds to appear? If not, and if I have misinterpreted that, then disregard the rest of this post --
--
I've worked with UE4 on a Navy simulation for carrier ships (it was basically a game) and yes, that is essentially how it worked. (I know that EU is CryEngine, but I'm 99% sure that, at least this aspect, is programmed exactly the same.) The world was divided into zones such that wherever you currently are, you are in "some particular zone". There is a list of linked zones for every zone... So if you're inside ZoneA, the engine knows that it needs to not only display ZoneA, but also B,C,H, and M. When you're in ZoneB, it needs to also load A,C,M,X and Y, etc. Typically those "child" zone lists are manually defined, because just rendering the immediately adjacent zones is not always satisfactory.
So for example in the Port Atlantis Mall, if you're in the main corridor on floor 1, then every single hallway leading to each shop on floor one is also loaded, BUT, the shop interior zones themselves are not loaded. Once you step into the entry hallway for any shop, then the shop's interior should load. And when you're fully inside the shop, the main floor 1 mall corridor UN-loads, and now it's just the shop interior you're in, and its entry hallway. To be clear, the main corridor, the entry hallway into each shop, and the shop interiors themselves are all separate zones. This is exactly how we did it, too. (Btw when I say "zone" I do not mean "instance". I just mean a grouping of meshes and items that can be loaded and unloaded as a single chunk.)
My guess as to why zones disappear sometimes is mostly due to server lag or client lag. You run quickly into a shop corridor and for whatever reason the game does not update fast enough and doesn't load the new required zone list fast enough, and you end up going through a wall or floor and getting stuck. If you manage to fall through the floor, now you're either in the "outside" zone (outside of the mall), or, the game just assumes you're still in the last zone that you were in just a moment ago... not sure which one EU does, I think the latter.
--
IN ANY CASE, the point of me describing this is I, too, am quite certain that this has nothing to do with the ability to drop items, at all. I have fallen through the floor in empty shops, too. It's just a case of the engine (or server) not being able to react quickly enough to your movements.
And once again if I've misinterpreted the "invisible floor/wall bug" to mean the wrong thing, I apologize.
As for the removal of dropped items, I'm incredibly upset about this and this might be the last straw for me.