I have a Sony 49XD7004 TV

When I play 4K HDR content from Amazon Prime (I don’t have access to other 4K HDR content yet),  the black bars at the top or bottom of wide screen (letterbox) programs are grey and not black (eg Goliath and Transparent) .  On normal 1080p, they are black.  This is slightly off-putting.  I've tried to change the settings, but they are still grey.

Any idea if this is a bug?