Is that a lighting group created through the smartthings app? Or something else?
It’s quite common for a device to be marked off-line but still to be reachable in any of several different ways. It gets marked off-line because it hasn’t checked in when smartthings expected it to, but that doesn’t mean it’s actually off the network. It’s just one of the annoying things about the old architecture. But since the old architecture will be replaced in a few months, I don’t think anybody’s spending a lot of time on it now.