Hasn’t the US clearly established the waters around Taiwan being international waters subject to FONOPs and explicitly not territorial waters outside of the 12km limit?
China and Taiwan are aligned in their EEZ/territorial claims in the region (all the way down to the Spratly and Paracel Islands), so these missile launches would constitute only firing missiles from one point in their EEZ to another point in their EEZ… Which, since as we’ve already established aren’t territorial waters, is perfectly legal in the rules-based system.