hmm you are correct, it only cares for the .logstash type. However, checking all my ES templates, I only have one template that applies to logstash-* indices. This template maps the index to a syslog type, yet I'm getting "Rejecting mapping update to [logstash-2019.08.28] as the final mapping would have more than 1 type: [doc, syslog]"
Do you have any pointers as to what would be causing this problem? Typically the "more than 1 type" error is when you have two templates competing with each other, but in this instance I only have the one.