error_log shows notices even though it is set to "error" or "crit"

I have setup Nginx on my Debian 10 server.
And in nginx.conf I have set error_log to error, which means it should only show errors or worse in the log (according to the docs).

Still my error_log gets filled with these notices:
2019/10/03 20:41:42 [notice] 25419#25419: signal process started

I even tried to go up one level from error to crit, and it still shows these notices.

How do I disable them?

Thank you!