You are supposed to show your return ticket with a tourist visa (those 3 months ones) upon immigration in Taipei, although they do not usually checked, as a condition of the visa. I was asked a few times at the immigration counter upon arrival in TPE from places other than HKG. The CX transfer desk always asked for the HKG/TPE/HKG ticket before giving my boarding cards to me. It is possibly better to carry a printout of all tickets when you travel.