Partially true,
If you leave the heat treated bullets out in the open, room temp., over the course of a year, they will approaceh 22BHN, but will not drop below that.
I store mine in a freezer at 0 degrees and after 2 years have not lost any hardness from what I first heat treat for.
Going from 30-35 BHN to 22BHN should not raise pressure enough to worry about, in my opinion. I have shot the same bullets with both BHN hardness and have noticed no difference. Not that there may be some scientifically measurable difference, but as far as accuracy, velocity, or case appearance, there was no difference.