See my previous post, that it's normal for any leading party in the so-called democracies to always perform worse on the following elections.
People simply get bored and vote someone else because they can. That system is made to be very cyclical in nature.
I live beside the EU, and I understand this system very well. What occurred in these Taiwanese elections is very rare. They should've already been out if we follow the norms.
It's incredibly successful for a leading party to repeat 3 election cycles, and still hold 7% lead (as of my knowledge, even impossible).
In reality, DPP should've made a much worse result than this.
It shows that either people are very in line with their policies, or the US has a very strong grip on this country's political system, to be this firm.
Unless DPP/KMT one day ends up having all the complicated international and local legal prequisites to run a country (which has about 0% chance of ever occurring), their word on what they consider a democratic process is not something you can take at face value.
We don't know how the votes are counted, who votes, or what their political legality is.
But generally, voting processes in East Asia don't follow the same pattern as ones in the west. They're much more stable, likely because the culture values the importance of completing a constant plan rather than switching around at the first signs of struggle. Also, East Asia has seen the rise from middle income to high income over just a few generations, while Europe is experiencing steady decline, so parties like CPC or LDP have huge legacy clout, like how Democrats and Republicans dominate fully, because they presided over the rise of America to superpower status.
Both China and Japan had the same party stay in power constantly ever since ww2.