A marker is sometimes missing when you have three or more items at the exact same location and two cluster groups are represented in those items. In other words, a cluster from one group and an individual marker from another group at the exact same location.

In some cases the individual marker will not appear at all, depending on the combination of actions that gets you to max zoom. If a marker does not appear, its info window will sometimes still open, such that you think you're clicking on a cluster from group A (that normally would spiderfy) and instead you pop open an info window for an invisible individual marker from group B.

To reproduce, assign three markers from two different cluster groups to the same coords. Position other markers in the area such that at min zoom, all three markers are a part of clusters. Two of the markers will belong to one cluster and the other marker will belong to another cluster, since it is in a different cluster group. Click on the cluster that includes two of the three markers so that you zoom in on that cluster. In some cases, even though the third marker is at the same location as the cluster of two markers, it will not be visible on the map.