onurbek:

I tap a tempo of 80 BPM, the little tempo led flashes in 80 BPM while I am tapping the tempo, but the unit does not change the tempo to 80 BPM and the little led keeps flashing at 120 BPM again.

At what point does the little led change back to 120 BPM ? If this is when youn next load a preset you need to check the Preset/Global tempo setting – see User Manual.