The Kotlin Docs contains the following information.

All nullable references to a are actually the same object because of the memory optimization that JVM applies to Integers between -128 and 127. It doesn’t apply to the b references, so they are different objects.

I wrote the main function and test code to check the above.
At this time, each result was different.

I test this using kotest.

class Ch7Test: StringSpec({

    "nullable Int between -128 and 127 should be caching" {
        val boxedA: Int? = 100
        val anotherBoxedA: Int? = 100
        
        assertSoftly {
            (boxedA === anotherBoxedA) shouldBe true
        }
    }
})

The test result was failed.

expected:<true> but was:<false>
Expected :true
Actual   :false

But when I test in main function, the result was different from the test results.

fun main() {
    val boxedA: Int? = 100
    val anotherBoxedA: Int? = 100

    println("debug: ${boxedA === anotherBoxedA}") // output was "debug: true"
}

Why is it the same logic, but the results are different?