Chinese citizens don't use their passport to fly to Taiwan, since neither side recognises eachother as a foreign country.

> All Mainland residents cannot travel to Taiwan on their passports when departing from Mainland China and must hold a Travel Permit to and from Taiwan (往來台灣通行證), colloquially known as Mainland Resident Travel Permit (大通證), issued by the Chinese authorities.

https://en.wikipedia.org/wiki/Visa_policy_of_Taiwan

So the five times I was on a plane from China to Taiwan full of PRC citizens doesn't seem to match your girlfriend's experience.

If we are using anecdotes, my Indonesian friend was blacklisted from Taiwan and was given no reason.