/data get entity @e[type=minecraft:minecart,limit=1]
The cart with a "player" passenger doesn't display this in its nbt data. See the attached screenshot. I am clearly a passenger of the only other entity in this test world, the minecart.
When it has a mob passenger, it properly displays that in its nbt data.
I have been troubleshooting a mcfunction that needs to detect a minecart carrying a player when I discovered the minecart doesn't "know" when it has a player passenger.
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Looks intended, Passengers does not store players. You could use the RootVehicle tag of players, but this won't work for 2 or more player passengers either, see MC-132587.
Same appears to be true for a pig with a saddle (see screenshot)
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