This is a mystery, but just to throw out one possibility.
For a number of years Shimano and others dabbled with roller clutches in lieu of the traditional ratchet freewheel mechanism. It's easy to recognize them by the lack of the familiar tick, tick, tick of the ratchet.
Roller clutches are fine, and are nice in that they run silently, but are limited in torque load. If overloaded they can slip causing what you're experiencing.
BTW- if your freehub ticks when coasting disregard this since it doesn't apply to your wheel.
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