With it potentially being an AVCS issue, I would hope that if that is the issue, an experienced technician would be able to pinpoint it via relationship between the camshaft position, RPM, and relative position of the AVCS cam phasing... or something of that sort.
Honestly, despite the training I've had, I really couldn't tell you what it may be. I just hope it gets solved so other XV Crosstrek drivers may know what HAS gone wrong.
"Why would I want to profess my feelings for some 3D girl?" - Tomoya Aki from the anime Saekano