我尝试了一些类似于你的代码。我甚至创建了一个像这样的测试scala proc：

create procedure test_proc()
    returns string
    language scala
    runtime_version = 2.12
    packages =('com.snowflake:snowpark:latest')
    handler = 'Procedure.main'
    as '

import com.snowflake.snowpark._
import com.snowflake.snowpark.types._
import com.snowflake.snowpark.functions._

object Procedure {
  def main(session: Session): String = {
    import java.io.{ByteArrayOutputStream, PrintStream}
    val outputStream = new ByteArrayOutputStream()
    Console.withOut(new PrintStream(outputStream)) {
        def maskColumnsUDF(maskChar : String = "*", noOfLeftVisibleChars : Int = 1, noOfRightVisiblechars : Int = 1): UserDefinedFunction = udf((attrVal :String) => {        
              var finalString =  ""
              if(attrVal == null){ attrVal }
              else if (attrVal.length > 4 ) {
                val attrArray = attrVal.toCharArray
                for (counter <- 0 to attrArray.length() - 1) {
                  if (counter < noOfLeftVisibleChars || counter > attrArray.length() - (noOfRightVisiblechars + 1))
                  {
                    finalString = finalString + attrArray(counter)
                  }
                  else {
                    finalString = finalString + maskChar
                  }
                }
                finalString
              }
              else { "***" }
        })
        
        val schema = StructType(Seq(
            StructField("column_name", StringType, nullable = false),
            StructField("featureName", StringType, nullable = false),
            StructField("distribution", DoubleType, nullable = false)
          ))
    
        val data = Seq(
            Row("A", "foo", 0.2),
            Row("A", "bar", 0.5),
            Row("B", "baz", 0.1),
            Row("B", "qux", 0.3)
        )
        
        var featureDF: DataFrame = session.createDataFrame(data, schema)
        featureDF.withColumn("xx",maskColumnsUDF("*",1,1)(col("column_name"))).show()
    }
    outputStream.toString()
  }
}';

而且效果很好。因为这是临时UDF，我认为你可能会面临的是，你正在使用不同的会话，这就是为什么你得到的错误，该函数不存在。
您可以尝试以下操作：

def maskColumnsUDF(session:Session,maskChar : String = "*", noOfLeftVisibleChars : Int = 1, noOfRightVisiblechars : Int = 1): 
    UserDefinedFunction = {
        val udf_name = s"maskColumnsUDF_${noOfLeftVisibleChars}_${noOfRightVisiblechars}"
        session.sql(s"drop function if exists ${udf_name}(varchar)")
        session.udf.registerPermanent(
         udf_name,
         (attrVal :String) => {        
              var finalString =  ""
              if(attrVal == null){ attrVal }
              else if (attrVal.length > 4 ) {
                val attrArray = attrVal.toCharArray
                for (counter <- 0 to attrArray.length() - 1) {
                  if (counter < noOfLeftVisibleChars || counter > attrArray.length() - (noOfRightVisiblechars + 1))
                  {
                    finalString = finalString + attrArray(counter)
                  }
                  else {
                    finalString = finalString + maskChar
                  }
                }
                finalString
              }
              else { "***" }
        },"@mystage")
    }

这将使用应在所有会话中工作的永久性UDF。我只是建议诊断错误。最好确定会话是否不同。