I did try to book this day last; its our last MK day so I booked my other MK days first, then jumped to the second half the of the trip to book epcot/HS/AK, then came back to do this day last. So yes, it could be that they are not letting me book the 10th day total because perhaps it thinks I have already booked all 4 of the days from the first half of the split stay.
Interestingly I just went and tried to book a multipass for the day AFTER our trip ends (when we wont even be there anymore) and it let me do it. So, I CAN book a 10th day, just not on day #3 when I want it. The system must be locking me out of the first half of my split stay.
I guess I'll have to call to fix it. But this is an interesting nuance that is likely related to the split stay.