It's a silly problem to experience.
I also bought on SingaporeAir.com a SQ outbound to BKK and TR back into SIN. I couldn't online check-in for the TR flight as my first name didn't match my KrisFlyer profile even though on SingaporeAir.com my details were pre-filled.
I raised it to the social media team who told me to contact them but I didn't bother and just did the counter check-in at BKK.