I posted the same thing on stackoverflow recently.
It seems it needs to be posted here too.

The problem is that you flag some content that is "flaggable" then the poster edits the post and your flag is incorrect and handled as incorrect.
This means you get an declined flag even though your flag was correct at the time you placed it.

Either mods have to look carefully for edits, or flags need to "die" after an edit, or the flagger get a notification and the possibility to retract the flag (on all types of content).

Link to related meta topic on SO: https://meta.stackoverflow.com/questions/384604/flagging-content-and-then-its-edited

Link to post I got a declined flag: https://superuser.com/posts/1441651/revisions
The first revision is in my opinion not an answer