I came across an unexpected behavior with Merge LOP when fed with SOP Create nodes in reference mode, where the references come out in a right-to-left ordering. My expectation was that they would be merged in left-to-right order, as when using sublayers. See the picture below for illustration.

After poking around I found that the SOP Import inside the SOP Create is behaving like a Reference LOP whose “Reference Operation” parameter is set to any of the “Append List” options. Maybe this is the reason? If so, would it make sense to add a “Reference Operation” parameter to the SOP Import, and expose it on the SOP Create?

Fantastic sample file. I suspect this hasn't come up before because I think it's relatively uncommon for a single prim to have multiple references added to is from multiple SOP sources in sequence this way. Or if it is done, I think usually there wouldn't be any overlap between the contributions of the SOP Networks (so the strength order wouldn't matter). Anyway, I think you're absolutely right that this should be controllable with a parameter on the SOP Import. While I'm at it I'll add the ability to choose between a Reference and a Payload (which I'f fairly suprised nobody has ever asked about).

That's great, thank you!

Fixed in 18.5.265.