Fixed in OpenLDAP 2.3.35:
Fixed slapd syncrepl delta-sync modlist free (ITS#4904)
I suggest you upgrade your master, given that 2.3.30 is already known to
have an issue with delta-sync. Also, rather than just stopping slapd and
running slapindex, you may want to stop slapd, go to the cn=log db, and
remove the index bdb files (keeping the database bdb files, like
id2entry.bdb and dn2id.bdb), and then run slapindex, so that the old index
files are completely purged. I'll also note that 2.3.38 is the current
stable release, so there are many other good reasons to upgrade. ;)