You should allow specifying path to a certificate chain. Pointing ssl_certificate at crt and intermediates does not work - it just grabs the first entry only.
This sounds like a genuine bug report in which case you're likely to get better traction if you file a bug here: https://github.com/logstash-plugins/logstash-input-lumberjack/issues/new
I wonder if what you're talking about touches upon what's going to be addressed in the context of github.com/logstash-plugins/logstash-input-lumberjack issue #31.