I’m sure that’s very frustrating. 
Based just on reports in this forum, I don’t think it’s a Phillips hue issue. There are many many reports of different Zigbee devices from different brands, even smartthings brand, going off-line. And there hasn’t been any change in the behavior of the Phillips hue sensors with other Zigbee coordinators, including an Amazon echo device with a zigbee hub inside. 
Smartthings has made multiple changes both to how their hubs handle Zigbee and to the cloud just in the last six weeks. So my guess is it’s most likely to be one of those things, again, you’ll see a lot of posts about Zigbee devices of other brands going offline in the forum.