This is generic info re Tight Bolt...

1. Rust or Loctite creates tight friction on the threads themselves. In this case, cutting the bolt's head off makes no difference, simply because the threads are bonded together.

2. Bolt tightened too much at factory.
In this case, cutting the bolt's head off instantly releases the clamping force, which is basically between the head and the surface that the heads touches.
The stud is now free to move.

I have had this issue (too tight caliber bolt) in many cars. Once the bolt is removed, it is obvious that there is no rust buildup.
So the cause for tight bolt was not rust but rather the impact wrench used at factory that might have been over torqued.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT