I’ve seen rapid changes at much lower altitudes, 3000-8000ft, for instance.
Again, it’s not the fact it’s changing that’s in question, that is normal, like where it might change from 29.96 to 29.97, and you may or may not catch the altimeter change slightly. I’ve seen the altimeter adjust by over 500ft on many occasions, indicating a large change in QNH.
Next time I catch this happening I will try to remember what it is set to before, and after I hit the B key.