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it's the last piece of the puzzle isn't it... it does spin very freely though?
i wonder whether the inertia of the slightly larger 34t cassette also has a role though. when you stop pedaling the cassette continues for a bit and causes the chain slap
swedeee-
I've run 42T cassettes for a long time on Di2 and we've both run road Di2 on gravel bikes. There can be some chainslap but you'd have to big fangin' in the smallest cog and suddenly stop pedaling. It's basically a non-issue. If it's a consistent problem it sounds more like the seal thing mentioned, or maybe something needs a clean - like you've over-greased the freehub or something and it's sticking a bit too much.
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Spins freely but have the tiniest bit of drag. I've done a few hope hubs services, put the rubber seal wrong = chain slap. It's the same with most brands.
If you're super lazy chug a bit of WD40 behind the cassette where the seal is, see how much better it gets (not a long term solution)
hippy
Netakure
It's almost surely the seal of the freehub dragging, nothing to do with chain length.