you weren’t cruising at 18,000ft were you, as pressing B when above 18k will set 29.92 and if the barometric pressure is 30.86 in that area it could easily change back to that when you drop again, which might happen just by setting 29.92, if you see what i mean?
I don’t cruise at 18,000ft now, because of this, so 17,000ft or 19,000ft, to be well away from the transition altitude.