As explained here, if a post is deleted through spam/offensive flags it is replaced by the message: "This answer was marked as spam or offensive and is therefore not shown - you can see the revision history for details".
However, this post (visible for 10k+ users on math.SE and ♦diamond users) was apparently flagged as spam and the flag was marked helpful. (One of the users who flagged the post confirmed that his flag was marked helpful in chat). Yet, the text of the answer is still visible, it was not replaced by the message mentioned above. (Here is link to revision history, which does not show any changes at all.)
What caused different behavior in this case? Is this a bug?
I will add that when this was discussed in chat this explanation was suggested by a more experienced user:
It seems that a mod did not apply their (binding) spam flag to the post but instead destroyed the user. This led to automatic deletion of their post, and validated the flags on it. But did not result in the content-censoring notice.
But I guess only mods or SE-developers can confirm whether this was the case. And from this case it is also unclear whether the same problem would occur for offensive flag. (But I guess the mechanism used in both cases is the same.)