I would like to share my experiences on 2 separate trips from SIN-BKK-SIN. On both trips, the departure out of SIN is on Sat and returning on next day, Sunday.
During the first trip, I was not issued the return BP from BKK-SIN as I was told that this is not possible.
On the 2nd trip (which is a month after the first trip), during check-in, I was issued both SIN-BKK and BKK-SIN BP without having me to ask for it.
I guess you need an experienced agent to print the return BP.