There were a couple economy updates that were running, failing and rescheduling themselves every hour for the last couple of days. The problem seems to be caused by them, when they run, then eliminate the staff bids made during their running time.
At the moment, that seems to be the cause, but I'm not 100% sure, even though this is an old theory of mine, which I could never prove (things never fail when you want them to!).
If true, it would explain the similar issues with staff in the past, mostly happening on Sundays and Mondays (when the economy updates run). I'm considering solutions, but ATM only thing I can think of is to postpone staff auctions during economy updates, which would result in a very slim Sunday night for staff...