Most Pangcah/Amis tribes live on the plains of Taiwan’s East Coast.
The confusion comes from people mixing the level of sinicization with the geographical location of the tribes. On the West Coast, Pingpu, a.k.s. Plains Aboriginals are generally more sinicized than those living in the mountains. So Plains Aboriginals on the West Coast became synonymous with Aborigines who have lost their tribal identity.
If we stick with the actual definition of plains and mountain, then I think there shouldn’t be an issue with categorizing Pangcah/Amis as Plains Aborigines.
In fact, for the elections, what we would normally define as Pingpu Aborigines aren’t included in the Plains Aborigines category, since the government don’t recognize any of them. That means Sirayans, Taokas, Makataos, Babuzas, and all the other Pingpu Aborigines don’t get to vote for an Aborigines legislator.
The issue then becomes why would the election even have a division for Aborigines based on whether or not they live on plains or in the mountains? Why is that even necessary?
Also, why do some Saisyat, Altayal, Paiwan and Rukai villages get grouped in the Plains Aborigines category? I think where they live can definitely be considered as mountain regions.