Just got off the phone with PayPal support about this issue (over an hour just to get someone on the line, and that's not unusual). The tech told me (and I'm not even joking here): "The sandbox is totally antiquated. Don't even use it. Just test live using a real credit card." When I told him that was not an option, he disappeared for a few minutes and came back and told me about this post (which I'd told him about at the start of the conversation). So apparently the official PayPal stance is "Just test live." We're looking for PayPal alternatives now.
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