After review by the development team, it has been determined that this issue is related to a known limitation with the software that lies outside of Esri's control. The issue's Additional Information section may contain further explanation.
Esri Maps for Office allows a maximum of 50 groups for a layer.
1. Share the layer to ArcGIS Online. Add to a new map.
Use the same 'Other' symbology from Esri Maps for Office (EM4O).
2. Click the drop-down arrow to the right of the layer name, and select Change Style.
3. Select 'Unique symbols'. Scroll down, and see 'Other'.
All the layers is grouped in the 'Other' group.
Manually drag them out if desired - or click the Ungroup button to take all layers out of the 'Other' group.