I've the same problem. Changing the live accesstoken of another paypal business account of mine the transaction successfully. In sandbox the transaction complete successfully. A customer of mine also called his bank and they said that is Paypal that declined the payment. Contacting the merchant support the said that: "When I reviewed your Braintree to PayPal connection, it looks like it is still under review, which can cause problems when you attempt to process PayPal payments." They suggested to contact braintree support, braintree support suggested to contact Paypal at another email address and they suggested me to change integration mode.
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