I wouldn't risk it.
The most reasonable interpretation of a return ticket after the visa has expired is that it's the same as having no return ticket at all. If you put the details into IATA (https://www.iatatravelcentre.com/passport-visa-health-travel-document-requirements.htm) you get a warning
Warning: Visitors not holding return/onward tickets could be refused entry.
I'm guessing that most check-in agents that see that warning will not let you board.
One way to get around that, would be to buy a refundable ticket out of Sri Lanka that departs before the visa expires. You can cancel that after you are in country. Qatar might still take issue with that since you have another return ticket with them. I don't think anyone can predict how this may play out.