When lineWidth is defined in point's marker options, border is drawn correctly as in version 4.2.7.
In version 5 point marker's lineWidth is not respected.
The problem might be with seriesTypes.line.pointAttribs() as the following:
In Line strokeWidth takes seriesMarkerOptions.lineWidth directly with omitting pointMarkerOptions.lineWidth
The same situation with lineWidth defined for a specific state, here
Fixed #5897, Series.pointAttribs ignored some lineWidth options.