InstancePlaceholder doesn't assign Resource to instance

[crapola]

Tested versions

These versions work fine:
v4.2.2.stable.official [15073af]
v4.3.dev1.official [9d1cbab]
v4.3.dev2.official [3524346]
v4.3.dev3.official [36e943b]
v4.3.dev4.official [df78c06]
v4.3.dev5.official [89f70e9]
v4.3.dev6.official [89850d5]

Bug appears with:
v4.3.beta1.official [a4f2ea9]

System information

Windows 11 - v4.3.beta3.official [82cedc8]

Issue description

Node instantiated via InstancePlaceholder is missing Resource assigned in editor.

Steps to reproduce

A node that is a scene root has an export var that defaults to null such as
@export var res:Resource
That scene is placed in the editor in another scene, marked 'Load as Placeholder', and there we assign a new resource to 'res'.
After using create_instance on it at runtime, we observe the assigned resource is null.

Minimal reproduction project (MRP)

instanceplaceholder_bug.zip

Run and read console.

[matheusmdx]

Bisecting points to #90306 as the culprit: