Seems problem occurs for any log_level..
As you can see here: https://github.com/shadabahmed/logstasher/blob/master/lib/logstasher/railtie.rb#L7
Logstasher initialized as instance of ActiveSupport::OrderedOptions:

config.logstasher = ActiveSupport::OrderedOptions.new

But in new rails it also inherit an ActiveSupport default logger, which sending output to stdout.

And after that.. we moving to setup method.. where we have already defined logger:

self.logger = app.config.logstasher.logger || new_logger(self.logger_path)

And now at any call of logger output sends to default ActiveSupport::OrderedOptions logger..

This can be monkeypatched as follow:

# add this info your development.rb or other environment file
config.to_prepare do
  LogStasher.logger = Logger.new("#{Rails.root}/log/logstash_#{Rails.env}.log")
end

+1 for this issue -- even with the monkey patch, I can't get my development Rails.logger.info to display anything in the logstash log.

Same here... Even with the ninja Monkey Patch, no logger calls is logged

You might want to check #108 .
There is still one open topic but for me it's minor and should be fixed in another subsequent PR.
Comments welcome.

Other then this I'd say get it merged soon as this is really a pain.