The only time I've had Marco Polo as a retainer, he was attached to an Admiral. The extra movement helped but the second benefit clearly suggested he shouldn't be going to an agent or Admiral (something like: enables your army to .... overland). Clearly a bug.
I've also had a Cardinal with the "secretly a female" trait but that's clearly deliberate imho. CA having a joke and I must admit I had laugh when I saw it.
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