We have threaded that needed since the beginning of the One China Policy, one that Reagan and the 2 Bush Presidents followed, FWIW.
China has never liked that we sell Taiwan weapons but they deal with it because (a) they are mostly older weapons; (b) they are mostly defensive weapons; and (c) China remains confident it could obliterate Taiwan regardless of the sales.
We agree with (c) so why do we do it? To extend some military and political influence in a region dominated by another power. TPP would have done the same thing.