I think the reason why Buttigieg probably improved upon his numbers post-debate while Harris and Castro dropped was that he didn't go after Warren. Sure, Warren is very well-liked among Democratic voters and is more popular now than Biden himself in favorability but many more voters might just have a stronger affection for Biden because they knew him for so long. Buttigieg smartly avoided this because his numbers could've dropped as well.
Also, I'm not sure if we can draw too many conclusions from this poll due to high amount of undecideds and Buttigieg was already polling around the lower double-digit range from other pollsters.
Here's the change from last poll:
https://twitter.com/daveweigel/status/1186258970480132097Buttigieg "surged" from the last poll, but the poll was taken in late June/Early July.
