Well, this is a fun one.
What is going on
It seems, unwrapping depends on your shader's image texture, not the texture in the UV editor.
Again. It's not about the UV editor, it's about the shader editor.
I have three textures with different aspect ratios. 1:1, 2:1, 3:1.
But the UV map ratio is always based on the image selected in the shader editor.
Your texture is distorted, because your image texture is in 2:1 aspect ratio, and your original UV map in 1:1 aspect ratio.
Either manually scale the UV map to half it's width.
Or use the Mapping node's Scale panel to correct for the distortion.