This is a question on MSE which I had answered a month or so ago. It was an interesting question and it took me some effort to write the answer. The answer got upvoted twice and accepted.
Two weeks later, for some reason the question got deleted. I flagged for moderation intervention and shortly after the question got undeleted by a moderator, who commented that he did so because the answer was 'clear and good'.
But then, two days later, the same group of users deleted the question again.
I wonder why? The question is certainly not off-topic. I can see that the OP did not include their own attempts on the solution. Still, the question was non-trivial, well-formulated and had an accepted answer. This means that other users could have been looking up for the same problem and find the solution immediately rather than asking it again on the same site. Basically, the only thing a deletion like this accomplishes is removing useful knowledge from the website!
According to the help center:
Before voting to delete, please check whether there are any good answers; if so, then the question should be flagged for moderator attention as a potential merge candidate. We don't like to lose great answers!
It seems to me that this rule was not respected at all in this case.