Yeah, this is a big problem, in my opinion.

Players abused diplomacy to spawn a lot of resources close to their starting point. There are obvious bonuses to having multiple resources connected to a single city (for example, you can get a ventri and a normal metal ore mine, or 3+ gold mines, or 12+base food every game). In essence, diplomacy aimed at finding new resources was a very good option when starting a game.

Then in some patch they changed it so that the new resources would always spawn outside your influence zone. I think they've set it to be at a minimum distance from any city you hold, even.

Unfortunately, the change means diplomacy is quite bad late-game, as the resources you spawn actually end up in enemy possession.

Only if the player is aware of this can he use this wonky mechanic to spawn these resources early by researching diplomacy, then expanding to them before the AI reaches them.

I far prefer the previous system, to be honest. As a fix to this problem I would recommend moving the "find resource" technology further up the tech trees, but allowing them to spawn in friendly influence again.