I just went through the exact same situation today. Availability was showing as 7 and I was looking to book 4 seats on TP using ANA miles but needed to spilt it into two pairs of tickets, in order to use miles from separate accounts. As soon as I booked the first pair the entire inventory was zeroed out. I was able to cancel and after the inventory reappeared I called ANA where the agent reproduced the same issue but insisted there were really only two seats available. HUCA and next agent was then able to grab 4 tickets and then split the reservation on her end. No idea if this is intended functionality from TP.