GE Links have a known firmware issue where they may occasionally lose track of the network. Nothing to do with SmartThings–take ST out of the picture entirely and it can still happen. Much discussion both in this forum and on pretty much every home automation forum there is. This is the reason for the yellow ! On the Hue Bridge. ( it’s also the reason why the GE links are not on SmartThings official compatibility list.)
It’s not every bulb every time, but it happens often enough to be noticeable. Last year I had 8 GE links and whether they were on the Hue bridge or connected to SmartThings directly at least one of them became uncontrollable every two or three weeks. (This was very noticeable to me because I have to have someone else physically deal with it.) We switched over to the $15 Hue whites attached to the Hue Bridge once those became available and never had any problem with those.
Some people will never see the problem, some will see it every couple of weeks, it’s just random. But it’s real. See the following for the least annoying ways of fixing the problem when it does occur: