Gridmap doesn't update in parent scene when "Editable Children" is enabled

[schertudi]

Godot version:
Occurs in both 3.1.2 and the latest release of 3.2

OS/device including version:
Linux - Ubuntu 18.04

Issue description:
I have a gridmap in a child scene, which exists as an instanced scene in a parent scene. I have "Editable Children" set to true here. However, when I add or remove meshes to the gridmap and save, these changes do not appear in the parent scene. If I disable "Editable Children" and save the child scene again, all changes to the gridmap are reflected in the parent scene.

I just need to get the paths to nodes within the child scene so I haven't actually edited any of the nodes from the parent scene (especially not the gridmap). Also the properties of other nodes update even when Editable Children is set to true. Because of that I would expect these changes to sync between scenes even if the instanced scene is editable, but they don't.

Steps to reproduce:
Using the minimal reproduction project:
-From the child scene, add or remove meshes to the gridmap and save.
-Switch over to the parent scene; note how these changes to the gridmap don't appear here.
-Set "Editable Children" to false.
-Switch to the child scene and save.
-Switch over to the parent scene again; all changes to the gridmap will now appear.

Minimal reproduction project:
gridmap-not-updating.zip

[KoBeWi]

This is most likely because the cell data is stored in a Map and they are compared by reference not value, so cell data will always "differ" from default in the instanced scene.

Somewhat related to #29221

[KoBeWi]

Still valid in 7e009a1