Welcome to our forum & thanks for the question!
The problem is that we do not have any collision-detection and hiding overlapping for custom markers, but I see that you've already reported this issue here: https://github.com/highcharts/highcharts/issues/19359
so one of our Map Developers will take a look at it.
For now I'd just recommend you use the native marker.symbol = 'mapmarker'
like here: https://www.highcharts.com/demo/maps/mappoint-mapmarker