Just to confirm you are running version: 2022.0.8 - I am sure you are but want to make sure?
As far as I can see, when you set eco mode on, the Thermostat sets the Target temperature to the current temperature.
Yes, it is mode dependant as to what it uses.
I think it would be useful to see some debugging of the inputs and outputs fro the Starling Home Hub.
In the main plugin menu, you can select one or more Nest devices to monitor using the
Fillter pull down. Just select one Thermostat and also set the
Logging Level - Indigo Event Log to be
Starling Home Hub Logging Messages. Then save and for good measure, reload the plugin.
You should then get logging messages related to the Starling Hub in general and the thermostat speciffically when you try out the operations you have mentioned.
You can send me the ouput via PM as it likely will include your API key etc.