When releases are being merged, the annotation of one of the discs being merged overwrites the target release's annotation. In mason, we merged all the current annotations.

e.g. I started by adding an annotation to the two releases I was merging:
http://test.musicbrainz.org/release/0481dc5a-bab1-4e0f-8d89-9b755711b170/annotation/452604
http://test.musicbrainz.org/release/126dd1be-f8d1-3a76-a97a-db35c921ea3b/annotation/452605

then merged the releases - http://test.musicbrainz.org/edit/14460137

Now the target release only shows the annotation from the disc that was merged: http://test.musicbrainz.org/release/0481dc5a-bab1-4e0f-8d89-9b755711b170

This seems to be rather destructive, because the target annotation is the most likely one to be correct. I would either expect the annotations to be merged like they were in mason (giving "Annotation 1. This is the target of the merge.\n\nAnnotation 2." or if we really can't do that (I don't see why not, given that we used to do it), then the target release should be the annotation we keep.