on the returns, there are a few AA, UA, and XY flights inexplicably sprinkled in. Don't get that.
You need to specify DL+ for
both airports (origin and destination). It sounds like you're only attaching it to the outbound.
However, it's far easier to filter out the flights you don't want if you use O:DL+, as
FlyingOnceMore describes, as you can easily filter out the codeshares on the results screens.