You can disable this behaviour via series.stickyTracking option set to false.
series.stickyTracking
false
Demo: https://jsfiddle.net/gh/get/library/pure/highcharts/highcharts/tree/master/samples/highcharts/plotoptions/series-stickytracking-false/
API Reference: https://api.highcharts.com/highcharts/series.line.stickyTracking