October 2020, SmartThings announced a change to the Alexa connection for Samsung SmartThings users. The change is to provide faster response times, support for more devices, and the ability to control multiple locations. If your Z-Wave devices are no longer responding to Alexa and you are connected through a Samsung SmartThings you will need to complete an update.
Samsung has setup a support page here:
https://support.smartthings.com/hc/en-us/articles/360046899371?mc_cid=330fb2da02&mc_eid=6b465411a0