I have tested virtual thermostat using your driver [ST Edge] vEdge Creator: a virtual device generator for end users - Devices & Integrations / Community Created Device Types - SmartThings Community and Rule to copy real (Zigbee) temperature measurement to virtual thermostat. It works.
Maybe there is a type problem when this GET HTTP type is string and the temperature measurement’s type is number. Type conversions are not possible using Rules API.