Ideas
Haven't seen a spawner being added to ZoneObjectData other then the snowglobes but I didn't test many other hoods.
Plants can be any kind.
It probably also happens with homework, instruments and other stuff if you do what's in this thread: https://answers.ea.com/t5/Bug-Reports/NEEDS-SAVES-Objects-stuck-when-placed-in-world-after-loading-a-r/m-p/14193914
Haven't had a chance to test this case.
You found some very valuable info. Currently in the process of smoothing up your guide and putting it in my blog so that's what I'm busy with :D
I think it also happen to wild prairie grass placed in the world of Chestnut ridge. Once you approach those patches of grass, the game lag severely and if you harvest them, it will just be almost infinite harvest... I got up to 99+ prairies grass and had so many nectars etc...