HomeKit cameras not syncing

This is a new problem, after a few months of faultless performance...

I have three HomeKit compatible cameras. Two outdoor Logitech Circle Views and an indoor EVE.

And numerous other sensors and switches, all using an Apple TV HD as the hub.

On iPhone, iMac and MacBook the cameras worked perfectly, but in the last few days each has started showing different thumbnail images in Home.

Instead of showing 'seconds' bottom right, they show 'now', which is not 'now' but an old, random image.

Below's one example, from left to right...

iPhone - an image from days ago

MacBook - an image from earlier today

iMac - an image from yesterday

• I've switched Home on and off in iCloud on each device, rebooted each device and then turned Home back on - no change.
• I've deleted and reinstalled the Home app on the iPhone, no change.
• I've removed and then re-added all my cameras - they will show up-to-date thumbnails for a while, and then revert to random, old, images.

All software and firmware is latest version.

Any ideas how I can see a live thumbnail for each camera again?

Thanks.