Polyline Tool Option has Fill function which doesn't work.

It works when doing a polyline on a vector layer. Best make a bug report, though.

I've tried this with the latest 4.2.0 pre-alpha appimage and Windows version and the Polygon Tool works fine on both paint and vector layers for colour fills and pattern fills.

Which OS and version are you using?

I'm sorry, I had a brain failure there. I confirm that the POLYLINE tool does not fill on a paint layer but does fill on a vector layer.