When I run
heroku logs, no log output is shown.
Apps that exceed a threshold of log output over a period of time can have their Heroku-provided log buffer disabled, meaning that only
heroku logs --tail (live log tailing) will display logs. Note that this does not affect external log drains in any way.
Since Heroku-provided log buffer often spans less than an hour of logs we recommend provisioning a logging add-on or configuring a log drain if you have a log server available already, for additional log retention.