737 Pre-stall buffet at high altitude. Not at landing config low alt. Why?

[Jet Man]

Would not suspect a simulator fidelity problem.

My understanding is that the stick shaker speed is calculated that way (ie below low speed buffet at altitude).

This is why the min manoeuvre speed is calculated differently - based upon stick shaker below approx 20,000ft and based upon low speed buffet above approx 20,000 ft.