Why [path-rewrite] default plugin logs to stderr

Hi loved community,
we are currently using kong logging in a special way about logging format and are wondering why there the custom logging is not respected by [path-rewrite] plugin.
Default logging to stdout we have switched completely of, and now we see log entries like this the following still in the container logs:

2021/11/10 01:03:19 [notice] 29#0: *83945787 [kong] handler.lua:18 [path-rewrite] originPath /installation-order/v1/buildingPartPlanningData/searches rewritePath /core/installation-order/v1/buildingPartPlanningData/searches, client: 192.168.5.1, server: kong, request: “POST /installation-order/v1/buildingPartPlanningData/searches HTTP/1.1”, host: “api-production.internal-network.x”

and this logging is send to “stderr”, what i completely do not understand.

Can someone tell what is going on here and where the code of this default [path-rewrite] plugin is located, that we can also take a look for that problem ?