I found that the not working device as "_" in its ID, something like ABCD_EFG, while working one has not letters and numbers.
I enabled verbose logging and I can clearly see that it's searching for the wrong device ID:
VeraConnectWWN::getChildDevices> ERROR: Found device #161 with Nest ID 'EFG', but no equivalent device found in the Nest data
The plugin uses a structure like nest_devicetype_id to store the Nest internal ID and I suspect it's something updated and breaking the parse. I tried to encode the values with no luck. If I remove the devices (both thermostat and humidity sensor), the WWN plugin recreates them, but then the same problem applies.
I opened a ticket with support and I'll update this thread when they'll reply in order to help others with the same problem.