I think it might actually be first come first served, when the 0 or the 90 are presented, the slots are available and they get persisted, when the 9 and 99 get processed, the slots are taken and the data doesn’t get persisted,
I’ve never checked, but I suppose this might actually vary across installs, I know the buffered-write prevents any values being over-written once persisted, but I suppose they do remain in the buffer until the buffered write occurs, so it might even depend when in the “buffered-write cycle” that they are processed. I wonder if redis plays any part in this too?