[syncrepl slave isn't reconnecting] Am I missing a setting or something?
First, I'm concerned that you might be confusing "interval" (used with refreshOnly) with "retry" (used with refreshAndPersist). This has been discussed previously on list, and is described in slapd.conf(5) man page.
With that said, your symptoms sound like ITS #4708, resolved in OpenLDAP 2.3.28. In short, set "retry", upgrade to 2.3.30, and try again.