I tried to make a household account booking for a CW one way flight requiring 50K Avios. I thought that would be fine with 38K points in my account and 18K in my wife's account. But the online booking system would only show me the miles+Avios option at 37.5K Avios (with an extra $350 in copayments) when I got to the payment page and the 50K option couldn't be selected.

Is this a known bug? Is there any alternative to calling up to make the booking?