arcgis desktop - How to position callout bubble leader using maplex?

I'm using data driven pages to produce a number of maps. I'm using Maplex to have a callout bubble labeling my features. The problem is that for about 5% of the features, the callout bubble is not touching the feature. I want the tip of the bubble to either touch the boundary or be within the polygon.

In the image below, the labels on the right and left (59 and 63) are correct, while the one in the middle is incorrect because it isn't touching the feature. Note, these labels are coming from the blue polygons, not the yellow line in the middle.

The relevant settings are shown in the images below:

I would think that having the anchor point set as the eroded centre would do the trick. However, it looks like it may be placing the label in the centroid instead.

Any thoughts?