18.104.22.168 Preview, 1.18.31
It appears that structure blocks aren't creating new entities when loaded. But steal the entities from the original location and any previous time the structure was loaded. The result is a very restricted usability for entities with structure blocks. When you copy a build with entities. the original build will loose it's entities on reload. If multiple copies are made any entities that were in the structure block will only exist in the latest placed structure block once you reload the area.
Steps to reproduce.
1. place down any entity you wish.
2. create a structure that includes the entity or entities using a structure block.
3. Load the structure block once or multiple times. (number of times won't mater)
4. Save and close the world.
5. Reload the world
All entities will remain, both the originally placed and the ones copied by structure blocks.
Any entity related to a placed structre will be deleted, except for the most recently placed structure. This includes deletion of the originally placed entities.
Entities that were not saved as a part of the original structure or placed by the structure will not be effected. Entities that are a part of different structures will only be effected by that specific structure not by placement of another structure that may have similar entities but not saved from the same original ones.
photos provided show the before relog and after relog of this issue. world download provided but can easily be reproduced in any world.