Hello,
I've never heard about this behavior before, nor have I encountered such on my own 3458A, especially not when I turn it off regularly, and switch it on again a short time later.
It's definitely no regular state of the instrument, and definitely nowhere documented.
You probably want to indicate, that you have frequent mains black- or brown-outs?
So, how long do these interruptions last, i.e. minutes, seconds, or are these just brief break-downs of the mains voltage?
I can only imagine, that latter events only may cause this behavior.
The calibration constants should be safe.
Frank