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@gbj_tester
The "softness" of the carbon steerer is, I think, the lesser part of the problem. Although the lack of crush resistance of a carbon tube does mean that that the clamping force has to be lower, the larger issue I think is that the very thin walls of the steerer clamping band of lightweight stems stretch under load, potentially reducing the clamping force close to zero on some sections of the clamp, which allows the stem to move up the steerer. As the force alternates, first the top of the stem moves up, then the bottom, which is what I call "walking". The problem is reduced by ensuring that the steerer tube projects through the stem clamp, necessitating a small spacer above the stem, and also by stems with a tall stack height. Short stems (e.g. BMX, DH) will obviously be less prone to the problem simple because the force on the bar has less leverage, the overall mechanical advantage being Effective Stem Length/Stack Height.
Effective Stem Length is the radial distance from the force vector applied by the rider at the grips to the steerer clamp. On a road bar, a riding standing up on a climb pulling on the brake hoods can have an effective stem length of over 200mm.