I'm also having this same exact issue (the first sentence in the original post). However, I have tried in both the iOS and Android apps. When I try to set the Heat-Cool temperatures, it seems like it doesn't save both. For example, I set the Heat temperature to X, then click the Cool tab to change it's temperature and set it to Y. But when I go back to the Heat tab, it still says Y, and the X setting is gone.
EDIT: I've found another post that states this is by design: https://forums.stringify.com/t/nests-heat-cool-mode/247/2
The heat-cool action will put it into heat-cool mode, but only change the temperature of either heat/cool depending which tab is selected when saved. The solution is to create two separate actions within the same flow, one for the heat temperature of heat-cool, and another for the cool temperature. The thermostat may not let you change both settings immediately after each other, so you may have to put a timer of a few seconds between the actions.
Really wish the app would specify this is how it works, because it is completely different than how the nest's own interface works.