copying my email response to you here:
The tap tempo and synchronised LFO are essentially two different functions and can operate independently if you patch in an external sync input. The LED shows the status of the tap tempo, where it goes bright at the start of the cycle, then gradually fades. Unless you've selected the alternative waveform, in which case it is reversed.
It didn't make sense to have the tap tempo light any other way, though I understand it could be confusing as it doesn't correspond to the LFO level.
hope this helps!