The following is the checklist for when an individual sensor is stuck… You probably already tried all of these things, and they don’t usually apply when it’s more than one sensor, but just for completeness here’s the checklist:
If it was just one sensor, I’d probably just assume it was a bad device and return it. It happens. Some estimates are that about 15% of inexpensive electronics are DOA.
My concern is that you’re now seeing the problem on more than one sensor. That’s much more likely to be a network issue, either interference or range.
Just to be sure… Is your smartthings hub at least 3 m from your Wi-Fi router? Just looking for local interference possibilities.
Also, look on the devices themselves and check that the model number is right.