Do phantom power supplies (just the single I/O devices) put 48v on the output pins?

If I don't misunderstand things you're talking about an external power supply, so you're not providing phantom power from the mixer? If so then the answer is no, an external power supply does not send phantom power back to the mixer as that probably would damage the mixer badly.
Tl;dr The phantom power never goes in to the mixing console, only out from it.
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