When saving a large amount of entites, for exmaple, more than 20, structures will not save all of the entities. This is both with the block and the command.

How to reproduce:

Give yourself a structure block and enable the "Include entities" and "Remove blocks" options.

Summon or spawn in a very large amount of entities, at least 20 or so, inside the bounding box of the structure.

Press "save".

Then, attempt to load the structure you have just created.

Expected result:

All of the entities are loaded in.

Observed result:

A random selection of the entities will be loaded instead, ranging from 0-~30, consistently.

That is, if you hit save on 100 entities, and only 5 were actually saved by the structure, you will continue to only see 5 loaded. The amount that is saved varies, but it is always significantly less than the actual amount of entities that were present when saving. Also, which entities were chosen to be saved seems mostly random, although that may require further testing to confirm.