From your description, I assume these:
- You bought both the base game and upgrade DLC from US store.
- You switched to SG account to play it, successfully (you can access contents of both the base game and DLC).
- However on the main menu, the game shows you an option to buy the upgrade DLC, which you already owned.
My theory:
- The game's coding is a bit lazy. Since you are using SG account to play, when the game checks for DLC purchase, it checks the SG PSN store, it checks whether your SG account bought the DLC. The right thing to do is to first check whether DLC is present/installed, regardless of region. If found (and it should be found), then mark it as purchased, no need to check PSN store.
- For this particular game, I see that both US and SG PSN versions have product ID "CUSA05770". So in theory the DLCs are cross-region compatible with the base game. (Not sure about the microtransaction DLCs, it's possible they are account locked.) Maybe that's why the game's coding is confused, cos it found that your account playing the game (SG account) does not own the DLC which it found on the store (SG store).
Bottomline, if you can play it and access both contents, just ignore the menu.