Did you do a full reboot of the HUB? Thumbnails are often cached, so if there is an exisiting one, it seems to stay in use untill the HUB is completely shut down (hold down red power button on remote for 5 seconds), and brought back up again. Putting it into standby mode may not work.
It’s rebooting. Not much to lose in giving it a shot. Your are not resetting any of your customization or deleting anything. It would be like unplugging the box