My nest item is as shown:
Number dining_room_target_temperature_c "Temperature [%.1f °C]" <temperature> { channel="nest:thermostat:c1c98214:xxxxxxxxxx:set_point" }
Yours doesn’t seem to have the tag - does that help?
Does it work via OH w/o Alexa? If so perhaps a workaround would be a virtul number item linked to alexa and linked to the thermo with a rule? Clumsy but would work.