It has just occurred to me that the reason for this may be because through Nest, Nest Eco can be On or Off independently while Nest Mode remains set to Heat.
So potentially switching Nest Eco Off via Nest registers in the ST app because the only correct value for ST app Mode is Heat.
However, switching Nest Eco back On via Nest may not register in the ST app because ST app Mode is already Heat, which remains true (and which would suggest the programming prioritises Heat over Eco).
The solution to this would seem to be that ST app State could correspond to Nest Mode, with values Heat or Off. The ST app Mode could correspond to Nest Eco, with values Eco or Eco Off.
Or…, I think, if the setting priority in ST app Mode were to be changed to Off, Eco then Heat .
I don’t know if posting this sort of thinking is helpful. If there’s a better approach to getting feedback to the integration team, please let me know. I’m new to this!