Hmm, that is a strange bug.
Maybe it's giving double the amount towards quest.
i wonder if you only harvested 1 bush what would happen.

i tested some stuff and seems like the pokecoins gain the bonus of Amulet coin key item if it is active when berry are harvested, disabling this should fix it.

It's not just amulet coin, but also AchievementHandler.achievementBonus()
https://files.catbox.moe/5vzj8m.txt

I might leave this for now, not sure if it's a bad thing that the berries money gains the bonuses 🤔

Thoughts on if this is actually an issue, or is it expected that the money gained would be affected by oak items and battle items?

I think it can be both good and bad. But the quest seems to give very low qp so it can stay this way i suppose.

This seems like intended behaviour, quests check how much money you actually gain, which is after multipliers.