Built-in scripts of child nodes in editor require editor restart to reload

[KoBeWi]

Tested versions

4.3 dev6 broken
4.2 works

System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 31.0.15.4633) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

Issue description

If you have a built-in script in a preloaded scene, and the script is attached to a child node, not root, and you also use the script as part of editor plugin, any changes to the script will be ignored and you need to restart editor to update it. This used to work in older versions.

Also this error prints when opening the affected scene while the plugin is active:
modules/gdscript/gdscript.cpp:741 - Condition "!p_keep_state && has_instances" is true. Returning: ERR_ALREADY_IN_USE

Steps to reproduce

1. Open the MRP
2. Go to Screen X
3. Press Test button
4. Open MainScreen.tscn
5. Edit the script inside Button
6. Change to a different text
7. Again in Screen X, press Test
8. The printed text is the same

You can run the same project in Godot 4.2 and the text changes.

Minimal reproduction project (MRP)

deadscripts.zip

[KoBeWi]

Seems to have broken between 4.3 dev3 and dev4.
I tried reverting #85501. It fixes the error, but not the reload issue, so it's something else.

[KoBeWi]

Bisected to 2e09985 ;_;

EDIT:
Also it's specifically from changes in resource_loader.cpp, not editor_node.cpp.