I know many confuses cavitation and ventilation at first yes, but I think bigcane is right on this one, as I have heard many of the early hydrofoil designers having trouble with this very issue, and as far as I remember Gunnar also have had these experiences, years ago, with different profiles.
Namely that some strut profiles will for some reason suck more air down, which can even translate into a more jittery ride as the rear stab can be affected from this air.
Have never experienced it myself, but I dont crank hard and push ha haa, too slow.
So yes, I believe this can happen, when going hard upwind, some foils have the strut lifting a bit (as otherwise the strut is normally a pretty neutral non-lifting device) which can make what you describe happen
And in tacks where you turn (yaw) it sharply the same can of course occur - air gets sucked down a bit on the strut creating some turbulence
Can not really help on this one, just say that I understand what you experience, and it is not cavitation, as this only happens on really bad profiles and usually at extreme high speeds only.