I already mentioned the one from the beginning of S5 as an example of Buffy doing something we're intended to see as against her own rules (also the ITW vamp ho). Also that one isn't staked while rising but after a chase and an extended fight back. The woman at the end of Sleeper is the last of a whole series of Spike victims who've al been driven to attack her en mass. It's not something she does all the time. It's something that's rare enough to be worthy of note given that you'd think it would be the most efficient way to get the job done. An absence that any theory that Buffy regards all vampires as stakeable regardless of how they behave, needs to account for.