I had saw a queer in the CHANGES of 2.4 branch, and also comment of aecec6a75.
Exactly: Fixed slapo-syncprov deadlock when autogroup is in use (ITS#8063,ITS#8081)
But really, this (ITS#8081) is NOT related to autogroup (ITS#8063). This bug was introduced by 7561998f7 (ITS#6335, Quanah Gibson-Mount quanah@openldap.org, 2009-10-30).
Yes, after ITS#8063 this bug could be seen as SIGSEGV. But before ITS#8063 the syncprov could `re-order` notifications of changes that are visible by a remote syncrepl. Under a highload (our use case) this makes possible to 'lost' a some changes by replication, and then spread this error to all nodes of ldap-cluster (I spent a lot of time to dig this).
So, this is a INDEPENDENT (critical) bug in syncprov (not in autogroup), which is also present in 2.4.40 release and so on.