The GE link bulbs have significant problems, which is why they were never on the official “works with SmartThings” list. It’s just a firmware issue where they can lose contact with the network every two or three weeks. Extremely annoying, so much so that I replaced eight of them once the $15 Hue White bulbs came out, and I’ve never looked back.
So it’s a known issue, two years later there’s still no fix, and you either live with it or get a different brand.
BTW, there is a forum FAQ on this issue. If you’re going to keep yours, definitely read the FAQ for tips on how to reconnect them more quickly.