In some instances the layer of an ArcMap Service will have a description, but will have no heading or title in the legend.
For example, you may render the U.S. states by region. The Layer has a heading - “US States”, a sub-heading - “Region”, and a description of each region class - such as “Northeast”. The sub-heading often describes how the layer is classified.
If the sub-heading ("Region") is missing the ArcMap Service will drop the heading ("U.S. States"). The legend of the ArcMap Service will only display the descriptions ("Northeast").
Layer headings are dropped when the layer has a layer heading and a description, but no sub-heading in table of contents of the original ArcMap document.