Not quite sure what you mean by "Long Tap Tempo", but I'm guessing that you are trying to set a very slow tempo, and the unit is timing out. The minimum tempo the tap system will accept is about 30-40 BPM.

Given that 120 BPM is typical, if you are less than (say) 30, it assumes you have stopped tapping. It can't wait forever for the next tap, so has to make some assumptions.