I agree with Robert – I think he nailed it.
My best detailed explanation for what I think is happening:
The electromagnetic interference is from tiny arcs produced when the static charge builds up enough to jump to another material. Think of static shocks, but they’re smaller than you can see/hear and jumping all over the place. The static charge is developed by dissimilar materials rubbing against each other (triboelectric effect, like rubbing a balloon on your hair), whether it be air and plastic or sawdust and plastic. With more dust thrown up by the upcut endmill compared to the downcut, you have a higher surface area for charge to accumulate and more opportunities as the dust moves for arcs to form between the dust and hose.
It sounds like you’ve done a good job grounding things already, but could there be static build-up inside your plastic shop vac, or even in your sweepy?
Do you live in a particularly dry region? One possible reason for variations day to day is the changing humidity. The more humid it is, the more readily the charge can dissipate into the air without arcing – or arcing at a lower potential, radiating less EMI in the process.