So an update (and my resolution). The SmartThings dev team looked into my hub logs and devices and found that the mesh was essentially losing packets because of some bad routing devices which causing some sleepy devices to be marked offline. They recommended that I rebuild my mesh, identify the routing devices that were causing problems.
That’s what I did, identified which routing devices (in my case it was a mains powered switch) was causing problems and rebuilt my mesh. After doing so, it’s fixed my issues and no more devices going offline and coming back online.
Some FLiRS and sleepy devices are more susceptible to routing/mesh issues and I know that FE599 is one of those. I would recommend that folks do the same, identify which of your mesh routing devices are problematic (some devices are more problematic than others, the link below identifies some of them). If you don’t know know which ones are, then start removing main powered devices (and doing a Z-Wave repair after each removal) until the problem disappears and once you have a stable mesh the problem should be fixed.
Here are some tips to identify bad routing devices and rebuild the mesh.