SHOULD has a deeper meaning than you might think:
SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
So unless there is a good reason, get() returns the same instance. If you ran into inconsistencies, would you mind to explain that a bit more? It might be a bug in expressive-skeleton or maybe one of the containers. As far as I know, instances are shared by default.
In case of zend-servicemanager you can configure if an instance is shared or not. So the developer is explicitly configuring the container to not share a specific instance. And I think that's also where the responsibility is: It's for the developer and not expressive to do that.
I haven't seen any share/unshare functionality in the Aura.DI docs. So I guess if you want the same functionality in other containers, you would need to create PR's to the other containers.
Luckily for us there are several containers available. You are free to choose the one you are comfortable with and has the functionality you are looking for. Enforcing all containers to have the exact same functionality is not what the PSR's are about, nor should they.