Yes. If one of the receiving slave devices goes out of range of the master transmitting unit, its icon will turn grey on the BLINK Hub app. When it comes back in to range of the master unit it will appear as its usual colour again. This also applies for group settings.
If you turn a device off or it loses power, it will also turn grey. If you no longer wish to see this device in the workflow, simply click on it and it will disappear. When you turn the unit back on, its icon will reappear.
If the master device is switched off, the group settings will be lost and you will need to reassign them. This is the same applies if you close the BLINK Hub app and restart it.