cesium: Labels don't show for individual entities in datasources when clustering is enabled

Issue

Entities won’t show a label if their owning datasource has clustering enabled

Reproduction

Open this Sandcastle example, slightly modified from the basic Clustering demo, then toggle the “Enabled” checkbox. Be sure to zoom in enough that some entities are not clustered. Observe that individual (non-clustered) entities show a label when clustering is disabled, but the label disappears when clustering = true.

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I’m not sure if this is a recent regression or not, as I’ve updated Cesium a number of times in the last 2 months and just started noticing the problem recently. In the example, the entities are created from a KML source, but in my application I observe the same problem with entities I create manually and control with a CustomDataSource.