Error when packing node from tree at runtime

[KoBeWi]

Godot version

3.x fe2f24c / 4.0 06c1b40

System information

W10

Issue description

When you have a node on scene and use PackedScene.pack() on it, this error appears:

E 0:00:01:0897   _ready: Condition "!is_ancestor_of(p_node)" is true. Returning: false
  <C++ Source>   scene\main\node.cpp:1882 @ Node::is_editable_instance()

It was introduced recently, the error didn't happen few days (weeks?) ago. Also doesn't appear if the packed node isn't inside tree.

Steps to reproduce

var scene = PackedScene.new()
scene.pack($Node)

Minimal reproduction project

No response

[rsubtil]

Bisected to fab88a810c (#49664)