Do your parents have super visas or TRVs? If they have TRVs then yes one-way tickets could create issues.... the airline may insist on return tickets, may not allow them to board..... or the immigration officer at the POE could ask for return tickets to confirm they are indeed short-term visitors. Also, one-way tickets work out more expensive.
I would recommend you cancel their one-way tickets. And, re-book refundable tickets for both ways.
Refundable tickets are a bit more expensive than non-refundable tickets, but you would be able to re-book their return flights if they want to stay for longer by paying an small fee or some airlines do not charge depending on the fare conditions of the tickets