I've come up with a few more critical details while trying to solve the issue.

1) If the other master is started first, when the local slapd starts, syncrepl connects and works fine. It only fails if the other master is started after the local slapd is running. 2) If I delete the specific olcSyncrepl attribute for one that is failing, and then add it back, syncrepl starts working fine. 3) After deleting and re-adding the olcSyncrepl attribute to make syncrepl work, if I stop and start the other master slapd, syncrepl starts failing again.