I have been trying to trace down when/by whom a particular pull request in one of our Bitbucket repos was declined (have a feeling someone "accidentally" hit the button). In doing so I found that the decline action was not recorded in the "Activity" tab for the pull request.
Going back and looking at other declined pull requests (about a half dozen or so), I found that there are some that clearly have activity items with the red "DECLINED" text and others that also have no indication when/by whom the decline was done.
So it looks like in certain cases Bitbucket is not logging the decline activity.
Please let me know if you need specifics of our repo for further debugging.