An ArcGIS Pro map service published with the Unique Value symbology and the All Other Values option displays an offset legend in Event Editor.

Solución alternativa

1. After configuring the map to be used in Event Editor, go back to the web map and uncheck the 'Other' option in the symbology of the layer.
2. Save the map.
3. Recheck the option and save the map again, and symbology comes in correctly.

Pasos para reproducir