When org-log-into-drawer is t and the log book drawer is open, using org-add-note closes the log book drawer.
I think it would be better if org-add-note left the state (open or closed) of the log book drawer alone. In comparison org-clock-in and org-clock-out do leave the state of the log book drawer alone, which is much more satisfactory. This is with org-20160718 from GNU Elpa (which is Org Mode version 8.3.4) on GNU Emacs 25.0.95 (from the pre-release tarball).