The Taiwan Relations Act (TRA) is an act of Congress that requires America to make weapons available to Taiwan so they have the means of defending themselves. What that means precisely is decided by the President and Congress.
Unfortunately, that's the best I could give to you. Not really intentionally trying to dodge the question. My comments are based on number casual exposure from my personally reputable sources which I don't have time and energy to sort out, as I am not a full time intentional law practitioner.
The best my comments can give is for someone who is genuinely interested in learning history to spend time to look for reputable sources.
HN for things outside of tech industry generally offers sharply lower quality. So you generally should not treat anyone here including myself too seriously.
So then the answer is no, those weapons companies are not breaking the law. They have been selling weapons to taiwan for decades, and they do not have to worry about international law.
Forgive me for being naive here but how does Taiwan buy weapons? Are all the arms manufacturers selling to them breaking the law?