This rig (SUV w/ deflector + Apex trailer) may be an instructive example.
From Hucho's compendium.
The Apex already has a generous roof edge radius in profile (side) view. In plan (top) view it has pretty sharp edges. Gilkison reported to Phil record high mileage. To my brain all this evidence supports sidewall splash as a likely problem that the rooftop deflector addressed.
It would be nice to see 3D streamlines from proper CFD, or tufted sides of the trailer with and without the deflector, to further demonstrate benefit. I suspect long term FE will show real gain. With real world variability, however, scientific confidence requires more trips, patience for us, and generosity of those involved to share.
Rock on!