EntityCluster broken when adding new entities

Hi,

We're trying to use the clustering functionality on a project I'm working on and I seem to have run into an issue with labels appearing when they shouldn't. We have EntityCluster.clusterLabels=true (the default), but we still see labels for the clustered entities appearing on the map. This only seems to happen when you add additional entities to a map that has clustering turned on. I created a quick prototype in sandcastle, (see below), to reproduce the issue click the 'inject' button once, and you should see everything work properly:
If you click the inject button again, you should see labels for all of the clustered entities (but no billboards):
It seems to be behaving as if EntityCluster.clusterLabels was set to false the second time you inject entities. Our current workaround is to turn clustering off and back on when we add entities, but it causes the entities to flash briefly, which isn't ideal.

Sandcastle example:
https://sandcastle.cesium.com/#c=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Operating System: Windows Linux

Start with the linked Sandcastle reproduction and the EntityCluster configuration, then trace what happens when the inject action adds entities a second time. The issue is done when repeated additions keep clusterLabels enabled without requiring clustering to be toggled off and on, and clustered entity labels no longer appear individually.

Written by the indexing model from the issue text.