Wow - a MK2 still alive and kicking! Fantastic to hear. :)
The BPM displayed on the screen is read off the audio; if the tempo of the track changes, the display will also. It is not in any way locked to the tempo slider hardware. So in essence, you're both right.
Sometimes we have seen where a track is measuring just at the tipping point to another BPM so it bounces back and forth -- if this is annoying, either put a piece of tape over the display, or adjust both decks just a hair to have the tracks at the same tempo but at another percentage.
There is always the possibility the tempo fader is failing, but if the hardware still operates properly, I'd say it's the issue above.