Q70R TV no 5.1 pass-through

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My Q70R (firmware 1406) does not appear to pass-through 5.1 LPCM audio on the HDMI0ARC port to my HW-Q700B/SWA-9500S soundbar/rear-speakers .

Any idea why this basic function is missing ?

I have tried updating FW to 1406.4, but the TV doesn't seem to recognise it as an update to 1406.

With the TV audio set in 'Surround' mode, the rear speakers simply duplicate the front speakers' audio, as with any TV audio regardless of spec.