American Express mixing charges from separate cards

AMEX user here for many years. What you are seeing is how AMEX downloads have always worked. One of your cards on your account (likely the first one you got, presumably your current "personal" card) is the "master" card on the account. When you download from this account, you get all charges for all cards on the account: your "master" and all additional cards you later add to that same account.

In other words, you have only one AMEX account. Each card on that account has its own number and can have its own login. Your "master" card always rolls everything up.

Use case: Your "personal" card is the "master" and you add additional cards for your spouse and each of your kids. Your "master" card login is used to see and pay the combined charges for all cards.

I said each additional card can each have their own login at AMEX, and that login only sees and downloads the charges for that specific card. Your "business" card fits into this category. This is useful if you make each additional card holder responsible for paying for his/her own charges. But the "master" account always sees all charges (and I think that's a useful thing).

Friendly suggestion: create a third new card exclusively for personal use. On many AMEX accounts, there's no additional fee to add new cards. You'd have at most three accounts in Quicken, with three logins:
1) Your current "personal" card (the master)
2) Your new "personal" card
3) Your current "business" card

Downloads to 2) and 3) only see the charges made on that specific card, so now they're separated. You'd stop using 1) for personal charges. In fact, with 2) and 3)'s charges separated out, you could choose not to track 1) in Quicken at all (i.e. don't add it to Quicken) since, I'm assuming, you're paying the bills for 2) and 3) out of separate funds.

Hope this helps. The thing to remember is that the "master" card always sees every charge on every card. It may be a pain to set up logins for each card, but that's a one-time thing, and in practice, individual logins gets you the separation of charges you want.