Indeed, sm_op == SLAP_MOD_SOFTDEL, as I inferred from syncrepl code, thanks for the feedback. Internal special modification types should be handled now in HEAD code. Could you pull it from git and test, please? Instructions here http://www.openldap.org/software/repo.html.
Looks good! I can add and remove users to groups in my setup without issue. Thanks for your quick response.