If you have a ssl_multicert.config without dest_ip= rules, i.e. requiring SNI negotiation to get a TLS session, then you can not configure the session ticket keys block, at all. Meaning, there's no way to share the keys across more than one machine.
I went down a bit of a rathole trying to fix this, but it's somewhat ugly. At the point of resuming a session, the SSL call back provides the 16 byte key-name, but the SNI name is seemingly not available at this point.
A possible solution is to change the lookups to always be on the 16-byte key-name, and keep a separate lookup table for the key blocks. This is in itself a little ugly, because the ownerships around SSLCertContext is a little murky. But it seems the cleanest, and definitely seemed to have been the intent from OpenSSL's callback signature.
Another option, which could not be done in the 6.x release cycle, is to remove the ticket_key_name= option from ssl_multicert.config entirely, and only have a single, global key block configured via records.config.