My ex-boss was a US-er guy who live in Taipei with his family. While everyone has full residence of ROC as per the wife being a Taiwan-er, he didn't bother with those documentations. There was a chat between us that he always being asked the same question, and somehow there's a waiver / acknowledgement between CX and him saying he will bare all the cost if he is refused to enter / deport and that's done deal since then. He mostly fly EVA so he didn't face the issue anymore.

I always do ex TPE and have been meeting the same issue on-and-on, what I do is I always have my ex-TPE ticket ready before I fly and show the return journey to HK check-in staff.