Alright, this has been resolved.
The problem was that my Progression 140 has noticeably deeper dremeled out bolt holes than all of the newer Unifoil wings I tested. The M8 30MM screws that Unifoil uses with the Katana for this specific wing were too long and bottomed out the adapter just slightly before fully tightening down on the fuselage, allowing just a fraction of space to allow for rocking room. The Katana mast did not have this issue as its bolt holes are slightly deeper and still allowed for the bolts to screw down all the way.
I switched to use M8 25MM bolts, which I immediately felt tightened down deeper into the bolt holes, seating the adapter properly into the mast socket. And there is no rocking anymore.
Shout out to @ProjectCedrus for the idea to test