Hi,
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
Kind regards!