The CFM56-7 is a reliable engine. Theoretically it is still possible that something from the intake or fuselage got ingested in the engine, but like you said it can be also be a problem caused by CFMI or the airline.The 737-800 was part of the NG version so this is predating the MAX. An engine fire issue should be on either the airline or the Engine maker. The Engine on the NG is the CFM 56 series made by CFM the Franco American joint. That series is also used on any number of Airbus planes and the Boeing P8, E7 and other military versions of the 737 NG line.
It can indeed be the case that someone forgot to close the engine cowlings properly during inspection.That again would be on South West airlines as they do the maintenance.