Okay so debuffs have what are called "frames" (it's kind of a bad term, they're more like categories). Nergal's debuff and ambush are examples of A frame debuffs. You can only have 1 A frame debuff applied at a time. Sniper shot is an example of a B frame debuff. You can have 1 A frame and 1 B frame debuff applied simultaneously, and indeed they do stack by straight addition (say ambush - A frame 20% and sniper - B frame 20% would indeed be 20% + 20% = 40%).
However what we're saying here is not about that. It is about overwriting debuffs. Since you cannot have 2 A frame debuffs at the same time, if you apply a weak one and then you apply a stronger one, the stronger one should overwrite the weaker one. However that is not what is happening. The stronger debuff overwrites the weaker debuff but only for 1 turn. The next turn it goes back to the weaker debuff.