Entity instance is not correctly initialized inside .save() method

[Enrico724]

          I just found this issue because I thought one of my repositories returned plain objects, where all the others usually returned class instances on `save()`. Turns out that in all other places I indeed called `create()` before `save()`. It looks like `save` returns an entity instance when it was given an entity instance, and a plain object when it's given a plain object.

Originally posted by @quezak in #4363 (comment)