I'm running into a possibly similar situation with hardware along the same lines and have a guess as to what's going on (but no clue why on earth it would be this way,
I asked about that in HVAC-Talk)
So I have an S30, with settings enabled for Aux Dehumidifier (I only have a furnace, no AC or heat pump; but I do have a Santa Fe 120H whole home dehumidifier wired inline).
It wasn't running quite right (furnace fan doesn't stay on while it runs, dehumidifier priority seems to be lowest even below "Fan On" which I always leave on) so I started investigating.
I found with with my multimeter that the DH output is "active low" that is to say:
When the S30 calls for dehumidify it goes to 0V, when it is not calling for dehumidify it is at 24V!
This was exactly BACKWARDS of what I expected.
This is not documented anywhere as far as I can find (S30 or furnace manual or wiring guides), so I think there is a chance that it is a bug in the firmware.
That said, mine is wired properly with a relay (just seems wasteful and dumb as it is though, but I could definitely be ignorant of a valid reason) so it does control properly, it's just that I need my furnace fan to stay on and it doesn't.
If yours is backwards you should observe it turning off when the dehumidify call comes, which it doesn't sound like it happening, but maybe the humidity is naturally lower? Or did you fix it some other way?