Just an update: the problem persists with the pipeline.workers cranked way down, too. I did find something similar to what I was seeing in an old github thread:
And I tried exporting the SINCEDB_DIR var as a part of the options in startup.options:
read -r -d '' PRESTART << EOM
export SINCEDB_DIR="/var/db/logstash-sincedb"
EOM
Also to no avail.