I never noticed this in the old days when all pipes were metal and have been wondering if replacing all this plastic with copper or brass would stop this problem.
Metal or plastic pop up drain.
Bathroom sink pop up drain.
About 1 of these are machining 1 are sheet metal fabrication and 1 are aluminum sheets.
Also the drain stopper plastic pipes are covered with black mold that i can brush out.
In this bench top demonstration i talk about the differences between a plastic pop up drain and a metal pop up drain i demonstra.
Choose pop up or grid functionality.
Bathroom sink drains are either pop up drains or grid drains with the difference being pop up drains can close allowing water to build up in the sink.