1200 - 6500 sounds like a bug. It most likely it is a fan defect. You can try to plug another fan into that header to see what it says.
I do have to ask about the fan splitter.
Is it the one that plugs into the H100i? If so, the H100i will report speeds in its own software and not the bios. The bios can see the H100i pump speed only.
If it is another fan splitter, please ensure that one plug has 4 wires and the other has 3, if not, 2 sets of fan speeds are being sent to the board. This can cause misreads for sure. To be 100% sure, you should plug a since fan into that slot to make sure the board is not the issue. I can post an image if you want to see what I am talking about, but it is generally the yellow wire(third one next to black and red). Many fan splitters come setup wrong, I always just remove the extra wire and its pin to avoid issues.
With any of these fan splitters(the Corsair one is not actually a splitter), you can only see the speed of ONE fan since the header is made for a single fan.
It looks like things are working right as you see one of the fans on the splitter, one other fan and the corsair pump(that should also be connected to a SATA power cable to power it.).
I hope this clears things up a bit.