Custom resource doesn't save its data after being modified

[rockgem]

Godot version:
3.2.3

Issue description:
So I have this custom resource as a storage for an array of items, but every time I change into any scene the data stored in that custom resource somehow disappears.

Steps to reproduce.
I have found a weird way to get around with it by having the custom resource being referenced to any singleton class. This way, it still holds all values in the array after I added something into it whenever I change between scenes.

[AnidemDex]

I had a similar issue for dictionaries too (AnidemDex/Godot-EventSystem#23) maybe is related to #55885

Not related, this issue tries to save a variable that is not exported, so the property is not marked as something that be can saved