Anyone else noticed this?

When I play a 4K HDR bluray, the local dimming doesn't work.  The setting is locked at Medium and can't be changed but it's patently not working.

Yes I know the backlight is driven to max in HDR mode but there's no reason local dimming shouldn't still work.

If the entire image goes black (such as after logos etc at the beginning of a film) then the dimming kicks in and the screen fades to perfect black but, if there's anything anywhere on the screen at all, the brightness level goes up across the whole screen, as if local dimming was disabled.

Oddly I only notice this from a 4K HDR bluray source, if I play 4K HDR content from Amazon Prime (such as The Grand Tour for example), dimming works fine.

This is clearly a bug and I've reported it to Sony but, as usual, their support is atrocious and they seem totally disinterested.