Media Footage changes with when pre compose, even if new comp is created with selected footage.

You have created a composition with a 2:1 pixel aspect ratio. You can see that in the Preview icon with the selected file:

Check and make sure that the original footage also has a 2:1 Pixel aspect ratio. If it does, then open the View Options using the three bars to the right of the Composition name in the Comp Panel.

If the footage is not Anamorphic, check in the Project Panel or by using the File/Interpret Footage menu, then open the Composition Settings (Ctrl/Cmnd + k) and reset the Pixel Aspect Ratio to 1:1.

I don't know if this is it but it looks like a problem with Pixel Aspect Ratio

In your screen shot the Pixel Aspect Ratio (PAR) of the Comp is (2.00) it's the number in parenthesis nextt to the dimensions.

Click on the original footage and compare this value.

If the original footage is a different PAR open the Comp Settings (Composition > Comosition Settings) change the setting of the comp.

If the PAR is the same in both:

It could be a preview issue, click on the menu button next the name of the composition and make sure Pixel Aspect Ratio Correction is enabled.