You gain money for winning battles in instant resolution mode and none for winning manually. Why? For what reason? Is that a bug or something?

 Frankly, the combat might be not only the most intricate and well thought out part of this game, but the most enjoyable one too. Why would I ever want to skip it entirely? I want to see my special units fight, retain as many as possible and have them gain veterancy levels how I choose. And I get rewarded only for surrendering these things to the whims and terrible decisions of the AI? What point is there, for instance, in picking the Spanish then? Or the clandestine scouting ability from the Siege Cannons technology?
It is probably not a high priority issue but if it’s not bugged then I don't understand the logic behind it.

Of course!

Sorry if the quality is low, but as you can see here, independent mercenary army grants a good amount of spoils after instant resolution.

Take their flag and defeat all their units in manual combat and they grant no spoils; notice the money total in top right did not go up. I should specify that spoils from wildlife work as intended for me, just other empires and Independant people tend to give nothing after manual combat.

Windows Steam current Version 1.19.3556.0 no DLC

Save file: Agrarians.ctr

I made this thread just yesterday, but I've noticed others discussing this topic a year back, and iirc manual combat has never given me spoils since game launch.

Hope I could help

yes, not getting any spoils has been really bothering me.  It feels extremely unrewarding to get nothing after winning epic battles.  Every time I manual a battle I get nothing.  and so far, the spoils from auto are too pathetic.   and on that, the spoils from ransacking are pathetic.  I honestly hate it.  I really hope they fix this, or there's a mod to improve spoils because it's depressing