Hi there Chris,
Someone else recently had a similar problem and posted their solution: Unable to modify roles or see logstash indices
Can you take a look at https://www.elastic.co/guide/en/logstash/6.x/ls-security.html and try following the steps for creating a logstash_writer role, logstash_internal user, and configuring Logstash to authenticate as that user? Please let me know if this helps.
Thanks,
CJ