Is an investment account jointly owned because it was funded from a joint bank account?

A foreign brokerage or securities account can be reportable. Ownership follows the account's legal title, or an agent or nominee holding it for you. Signature authority follows who can direct the institution to pay out or transfer money or assets from the account; authority only to buy and sell investments within the account isn't signature authority. The source of the money doesn't decide either.⁠^{[3]Source 3}⁠^{[1]Source 1}⁠^{[2]Source 2}

A couple transfers $15,000 from their jointly owned foreign checking account to a foreign brokerage account in spouse B's sole name. The checking account is joint. The brokerage account isn't, even though both spouses' money funded it.⁠^{[1]Source 1}⁠^{[2]Source 2}

Check the brokerage agreement for the named owners and anyone who can withdraw or transfer money or assets out of the account. A person who can only place trades doesn't have signature authority through that power. Report each foreign account on its own facts.⁠^{[1]Source 1}⁠^{[2]Source 2}