When saving a structure, you have the option to "Remove Blocks" from the structure being saved, meaning that it will only save entities to the structure, and therefore when loaded, will only load entities. This works for 1x1x1 structures with entities, such as items, and same goes for mobs.  
But if you try to save a 50x10x50 structure with entities (such as zombies) within it, but enable "remove blocks", it will not properly load the entities upon loading the structure, if at all. Sometimes it will summon one or two of them for some reason, but most of the time, it will fail at loading the entities entirely. 

However, if you save blocks to the structure (where you do not enable Remove Blocks) it loads the entities just fine, regardless if you load the blocks when loading the structure. (Such as enabling Remove Blocks when loading the structure, it will load entities just fine, and exclude the blocks) 

This was found out as I was trying to save structures of entities to be able to summon a wave of monsters in an arena, using only one command to load a single wave of monsters. 

So there is a current "workaround" by not removing blocks from the saved structure, but that's still a bug as far as I'm aware, and not intuitive as to why it doesn't load entities when blocks are removed from being saved in a large structure.

Video proof: (as a one minute video went over the 10MB limit here, I had to upload to imgur..) https://i.imgur.com/vCv29n6.mp4