This seems to be similar to this:
Different J availability on RT vs one way ticket
It's more complicated than a POS restriction because in the GDS, I see the flight as J0 no matter how I query it. Wonder if UA is testing more advanced inventory rules only applicable to direct channels. Curious if a more experienced GDS user can get 872 to show J space.
Matrix prices the round-trip returning in D so I don't think it's NDC, must be something I am missing.