Godot handles scene like it's a different scene

Edit:
During runtime the game now works correctly. I could’ve sworn it didn’t. I have no idea what happened here. The visual bug in the editor still persists. I’m gonna leave this post up in case anyone stumbles upon the same issue.

Hello!
Recently, I stumbled upon the weirdest bug I’ve ever seen and I’m really confused now.
Look, I have a simple export for a PackedScene like this:
@export var next_level: PackedScene
And now, I’m trying to set it in the editor to level_6.tscn.

Easy enough, right? Well, no.
You see, Godot treats my level_6.tscn file like it’s a different scene, specifcally main_menu.tscn.

Look, here’s what level_6.tscn looks like:

But in the preview for it, it shows this:

As well as in this preview:

And for reference, this is what my main_menu.tscn looks like:

Now, that alone might be a funny visual bug.
The problem however is that during runtime the game treats level_6.tscn like main_menu.tscn, which of course breaks the whole game.

I don’t really know anymore what to do.
I’ve tried restarting everything, resetting the variable, deleting the .godot folder, but nothing helped.

System info:
v4.4.stable.official [4c311cbee]
Godot v4.4.stable - Windows 11 (build 26100) - Multi-window, 1 monitor - OpenGL 3 (Compatibility) - NVIDIA GeForce RTX 4070 Ti SUPER (NVIDIA; 32.0.15.6094) - AMD Ryzen 9 7900X3D 12-Core Processor (24 threads)