It seems that the thermostat integration has been updated in the Alexa app. I assume this has to do with the new smart thermostat by Amazon that was recently released.
The issue with this update is that it seems to expect a thermostat mode to be defined. If such thermostat only includes setpoint capabilities, as per the discover example below, it assumes that the thermostat mode is off, per the screenshot below, and the whole integration is disabled in the Alexa app.
Prior to this update, you were able to change setpoints even if the thermostatMode property wasn't defined. As a matter a fact, the mode wouldn't be displayed if it wasn't defined as a supported property.
{ "endpointId": "Thermostat", "manufacturerName": "manufacturer", "description": "description", "friendlyName": "Thermostat", "displayCategories": [ "THERMOSTAT" ], "cookie": {}, "capabilities": [ { "type": "AlexaInterface", "interface": "Alexa.ThermostatController", "version": "3", "properties": { "supported": [ { "name": "targetSetpoint" } ], "proactivelyReported": false, "retrievable": true } }, { "type": "AlexaInterface", "interface": "Alexa.TemperatureSensor", "version": "3", "properties": { "supported": [ { "name": "temperature" } ], "proactivelyReported": false, "retrievable": true } }, { "type": "AlexaInterface", "interface": "Alexa.EndpointHealth", "version": "3", "properties": { "supported": [ { "name": "connectivity" } ], "proactivelyReported": false, "retrievable": true } }, { "type": "AlexaInterface", "interface": "Alexa", "version": "3" } ] }