There is a known issue with GE link bulbs which can cause this. It’s the reason they are not on the official “works with SmartThings” list.
I haven’t heard of a similar problem with the Cree unless they got out of range. If the problem is that the GE goes off-line and then you are losing the cree, that would make sense if the GE had been acting as a repeater for the Cree. If the Cree is off-line by itself, then I would just wonder about range.
But in either case, try the steps in the FAQ. It’s quite likely you won’t have to change any of your smartapps, so it’s a lot less work.