Last week I noticed that first, my motion sensors stopped working and later the buttons too.
I changed the batteries for all of them and they remained offline.
I tried erasing them from the app in order to add them again but they would not connect.
I reset all the devices, I turned off and on the hub, I went to around two feet from the hub to try and make the connection, to no avail.
I have less than 10 zigbee devices and less z-wave devices connected.
From my checking, it looks like all zigbee devices (smartthings or not) are having trouble connecting, but not only zigbee.