It is allowed for citizens to enter Taiwan on a foreign passport. By doing so, they are then bound by the conditions of a tourist. Cannot work, must leave when tourist visa time expires. Lot’s of Taiwan citizens who moved abroad as children enter Taiwan on foreign passports and are not required to do military service. My son has entered on both his Taiwan and foreign passports.
Discussed here
And here. Taiwanese citizens can not only enter on a foreign passport they can also have ARC’s, just not APRC’s