Embedded game window does not support the same stretch settinigs

Godot Version

4.4.1.stable

Question

My project consists of a 2D game world with a camera rendering for a target resolution.
When the game is started, two instances of this scene are attached to two independent Window-nodes. Both have the stretch settings mode=“Canvas item”, aspect=“Expand” because I would like to have the most important part focused all the time and no black borders.
Here is what happens:


However, when I enable Viewport.gui_embed_subwindows on the main viewport in the project settings, I get two embedded subwindow which behave completely different. Their stretch behaviour is not understandable, even though I set the Window nodes to the same attributes as the main Windows settings in the project setting. This picture shows, that the windows do not understand which aspect to focus correctly initially:

Is this a bug? Or am I not seeing a setting? It is very confusing.