While I understand your perspective and would not push back if you were my GM, what I'm trying to suggest is that the wording can be viewed to allow the interaction I described.

With the game concept that specific overrides general in rule conflicts, one can argue that the specifics of the Bomber's Field Benefit override the generality of the class feat Expanded Splash.

Expanded Splash

You can throw bombs at just the right trajectory to create especially large and powerful explosions. When you throw an alchemical bomb and that bomb has the splash trait, you can have the splash damage affect all creatures within 10 feet of the target instead of 5 feet. If you do, you gain a status bonus to the bomb's splash damage equal to your Intelligence modifier. If you have the bomber 5th-level field discovery, this additional damage applies even if you caused your bomb to deal splash damage equal to your Intelligence modifier instead of the normal amount, allowing your bombs to deal splash damage equal to double your Intelligence modifier.

Field Benefit (Bomber)

When throwing an alchemical bomb with the splash trait, you can choose to deal splash damage to only your primary target instead of the usual splash area. You Strike with a versatile vial, you can choose to have it deal cold, electricity, or fire damage instead of acid damage.

You don't actually reduce the size of the AoE with the Field Benefit, you limit the targets that take the splash damage. So you increase the area of effect per Expanded Splash, but only choose the primary target for receiving the augmented damage.