The Alter Field tool does not honor the 64 character limit for file geodatabase feature class field names, and displays the following Error 000313: "The length of Field Name must not be larger than 31."
This defect is being closed as non-reproducible as the steps/data needed to reproduce the issue are no longer available. If you have a reproducible case, on a supported configuration, please contact Esri Support to reopen this defect.