I don’t have these ones in fridges etc yet, there is one at Site A just now which is doing exactly the same (worse actually, NS3 in the screenshot) but the code is out of date on that one, i’ve since added retries and waiting for the network etc, it’s sitting in a multideck which is an open sided fridge with shelves and the rssi is showing as -50+/-.
Still waiting on my prints to finish so i can move a couple of updated ones down there and remove the NS3 that’s there just now. In the mean time i have flashed 1 with just a battery and cap but updated code so it doesn’t sleep at all and just sends the temp every 5 minutes. The network is from a Unifi ac pro on the roof about 10ft away from the multideck (same where the test sensors are just now), you’ve got me thinking about the 5Ghz though, network is the same name, I’ll check Unifi just now and see what’s been happening and lock one to 2.4Ghz only (will do that with all of them eventually) to see if that’s the issue. Can’t believe that hasn’t occurred to me before you mentioned it 
thank you for helping