Since a few days I am over 10K reputation, meaning that I have access to the review tools. Up to now I saw 4 cases of "not an answer" flag there, which didn't make sense to me. The last one affected this answer of our moderator robjohn, which clearly is an answer to me. So I flagged it as "invalid flag".
In all 4 cases my flag was disputed. In the case of the answer of robjohn, he disputed it himself (EDIT: it was in fact disputed by the moderator Arthur Fischer, see his answer below) with the comment "a moderator reviewed your flag, but found no evidence to support it". However, his answer is still there. Strange.
After searching on meta, I found a few discussions indicating that the "invalid flag" flagging reason doesn't work as expected. Now i wonder why I should flag anything as "invalid flag" at all, when in the end it always is disputed counts on the negative side of the helpful flags.
So my question to the >10K users is: Is it possible that an "invalid flag" counts as being helpful in the end? Do you use the "invalid flag" flagging reason at all?