ADDITIONAL SYSTEM INFORMATION :
Tested on Windows and MacOS with Java Version 21.0.7 + Nashorn 15.7 (Maven). Worked with Java Version 11.0.27 (Nashorn included).

A DESCRIPTION OF THE PROBLEM :
When I create an empty array and push a java.lang.Long value, the value gets converted to a java.lang.Double.
When I create an array with an initial java.lang.Long value, the value stays a java.lang.Long.

This happens since we upgraded to Java 21 + Nashorn 15.7 (Maven Dependency).
With Java 11 in both cases the values stayed java.lang.Long.

I provide simple Javascript Code to demonstrate the behavior.

var testArray1 = [];
testArray1.push(new java.lang.Long(1234));
// this results in a java.lang.Double Entry
java.lang.System.out.println('1: ' + testArray1[0].getClass());

var testArray2 = [new java.lang.Long(1234)];
// this results in a java.lang.Long entry;
java.lang.System.out.println('2: ' + testArray2[0].getClass());

REGRESSION : Last worked in version 11.0.29

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute given Code with Nashorn Engine.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Both entries of both arrays should be of type java.lang.Long.
ACTUAL -
When creating empty array and push entry, type is of java.lang.Double.

---------- BEGIN SOURCE ----------
var testArray1 = [];
testArray1.push(new java.lang.Long(1234));
// this results in a java.lang.Double Entry
java.lang.System.out.println('1: ' + testArray1[0].getClass());

var testArray2 = [new java.lang.Long(1234)];
// this results in a java.lang.Long entry;
java.lang.System.out.println('2: ' + testArray2[0].getClass());
---------- END SOURCE ----------