I’ve been dealing with this problem of Sengled light bulbs disconnecting repeatedly from smartthings hub as well. I did read on the Sengled website (https://support.sengled.com/hc/en-us/articles/360020226334-Using-Samsung-SmartThings-with-your-Sengled-bulbs) that you need to turn off Security mode. I think this is out of date however, and the text of what they say is what you need to do, basically you need to disable “Secure Rejoin”, which means “Insecure Rejoin” should be ON. A lot of confusion I know, given apparently the changes that have occurred in the Smartthings app.
I am uncertain if this will fix anything, but I did finally find this feature in the Smartthings Classic app. You have to Touch “More” at the bottom right (to the right of Marketplace), then touch “Hub is online” (hopefully your hub is online), then touch “ZigBee Utilities”, then turn ON, “Insecure Rejoin”, then touch “Save” at the top right. These instructions work for the Smartthings Classic app as of 7/20/2019. Who knows if they will be the same two days later.
Once I did this, I still had to go throughout the house and re-add the Sengled bulbs via that on/off 10 times quickly routine. They are all back on now though. I have a pretty strong mesh in my house with 10 repeater outlets as well.
I actually had rebuilt my Zigbee network and devices about a week ago before today there were massive drop offs of Sengled bulbs again, so I will probably not call it “fixed” until at least a month has passed. But I wanted to pass along what I saw at the Sengled site and how I found the option in the Classic Smartthings app. I also have (and always had) Device Health shut off. So my experience is that while turning Device Health off helps, it does not fix this problem in and of itself. There is a help option on the same page as the “Insecure Rejoin” switch that goes into an explanation of the pros and cons. But unless you plan to ditch all your insecure zigbee devices, the cons probably don’t matter much.