You're right, this clearly is a problem.
I believe that it stems from the overlapping of many dataLabels from different series.
Take a look at these examples:
1. if we disable the dataLabels on the first series then the clusters appear correctly as no dataLabels from other series overlap the clusters: https://jsfiddle.net/BlackLabel/otbrewcj/
2. if we set allowOverlap: true
on the first series the clusters labels also show up: https://jsfiddle.net/BlackLabel/nfo86705/
These demos may point you to some kind of custom solution that fits your needs while the bug persists.
Please report this as a bug on our GitHub repository: https://github.com/highcharts/highchart ... .md&title=
so our Map Developers can take a look and start working on it.