According to the File Geodatabase Size Limits document, a field name can be up to 64 characters long:
Field name length: 64 characters
I have a standalone fGDB table containing a column with a name 38 chars wide. The field's alias is the same as the field name.
I add this to an ArcMap document containing a single polygon layer, and publish the MXD to ArcGIS Online. The ArcGIS Online REST endpoint for the standalone table shows that the field name has been truncated to 31 characters long.
Is this a bug with ArcGIS Online, or is there a workaround?