See screenshot. This is a Galactic War game. If you hover there, you see "SHARED METAL: 9" Interestingly enough, I should be at -11 metal, but apparently I'm at -2? Am I magically getting 9 metal from my subcommanders? Same thing can happen with energy. I can't find a lick of mention on this topic. Please note, I am not asking about the "Shared Army" setting in a typical multiplayer game. I know what that does. This seems like some sort of scheme for unshared armies to share metal, except I only see it happen in a game with AI subcommanders????
You're right, even unshared armies share excess resources (metal and energy). If you are wasting a resource, it will be split between your allies who don't have full stores of that resource. When it comes to dynamic alliances, you have the option of sharing excess economy, as you may or may not want to do that.
If you are aware of it, then you can turn everything off for a split second to achieve a split second of storage, then turn it on. I believe if you started to get the gift, then storage depletes, you continue to get it. If that is not the case and you stop getting it, the tougher way is to turn everything off and just turn on what wouldn't send you back negative, which including the gift eco is more than you actually produced to begin with.