My ownership experience with Rapid Rise was bad - with age the mech (identical to yours) would ghost shift under load. The problem is that only the spring stops it from going to a smaller cog (which it wants to do under chain tension) unlike a conventional mech when the shift cable tension prevents it.
My suspicion is that it's the cassette which is worn in a few gears, but I binned the mech, returned to a conventional one and the ghost shifting disappeared immediately.
Rapid rise is a fundamentally flawed idea - it works in the showroom, but when the spring ages it becomes useless.
I suspect you're right
The cassette is new as is the chain and all correct
The hubs are good and only thing I can point to is the mech as no amount of experimentation solves it. In the "lab" all is fine, on the road or trail, slippage and only on certain cogs .
I will scrap the mech