All the mandolines(and one twelve-string guitar) that I have ever played have caused me loads of staying-in-tune-problems. Whereas the guitars rarely do. Now, the facile explanation is: you just notice it more with unison/octave strings. Which makes sense, but I think there is more going on than this.
    My theory is it has something to do with resonance. Systems vibrating with the same(or very similar) frequencies can exchange energy. We all know about soldiers breaking step while crossing a bridge. This is to stop the marchers transmitting vibrational energy into the bridge, which can result in catastrophic failure. Can it possible be that energy transfer between adjacent strings tuned the same could give rise to some sort of temporary extreme movement, or increased tension, which de-tunes the string? Just a thought. it's one I've had before, but haven't the technical knowledge to figure out if it's possible.