This will be fixed in the 1.15.3 release.
In the meantime you can work around the issue by adding explicit facetAxis.title.textStyle.fillColor
setting:
facetAxis:{
size: 50,
title: {
enabled: true,
textStyle: {
fillColor: "#000"
}
}
},
http://jsfiddle.net/f99waL6o/5/