Spring Boot 2.6.4 -> 2.6.6 : strange NullPointerException within Logback when logging a mock Exception

while upgrading from Spring Boot 2.6.4 to 2.6.6 , one of my tests (written in Kotlin), fails :

    @Test
    fun shouldLogProperMessageIfNotAbleToHitAPI() {

        val configValidator = ConfigValidator(GitHubCrawlerProperties(SourceControlConfig(url = "someIncorrectURL",organizationName="someOrg")),mockRemoteSourceControl)

        `when`(mockRemoteSourceControl.validateRemoteConfig("someOrg")).thenThrow(NoReachableRepositories("problem !",mock(Exception::class.java)))

        val validationErrors=configValidator.getValidationErrors()

        assertThat(validationErrors).hasSize(1);

    }

the build passes with Spring Boot 2.6.4. It works in Spring Boot 2.6.6 when I run the test individually in my IDE, but fails during the maven build.

the stacktrace was not showing by default, but after surrounding the call by a try/catch, I am able to get it, and it points to Logback :

java.lang.NullPointerException: null
        at ch.qos.logback.classic.spi.ThrowableProxy.<init>(ThrowableProxy.java:99)
        at ch.qos.logback.classic.spi.ThrowableProxy.<init>(ThrowableProxy.java:89)
        at ch.qos.logback.classic.spi.ThrowableProxy.<init>(ThrowableProxy.java:62)
        at ch.qos.logback.classic.spi.LoggingEvent.<init>(LoggingEvent.java:119)
        at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:419)
        at ch.qos.logback.classic.Logger.filterAndLog_0_Or3Plus(Logger.java:383)
        at ch.qos.logback.classic.Logger.error(Logger.java:538)
        at com.societegenerale.githubcrawler.ConfigValidator.getValidationErrors(ConfigValidator.kt:48)

Logback version doesn't seem to change, I still get v 1.2.11 .

Looking at Logback source code, in ThrowableProxy :

        if (GET_SUPPRESSED_METHOD != null) {
            // this will only execute on Java 7
            Throwable[] throwableSuppressed = extractSupressedThrowables(throwable);
            
            if (throwableSuppressed.length > 0) {
                List<ThrowableProxy> suppressedList = new ArrayList<ThrowableProxy>(throwableSuppressed.length);
                for (Throwable sup : throwableSuppressed) {
...

note : I build with Java 11, so the comment saying in Logback source code that this will only execute on Java 7 , seems wrong.

It seems that throwableSuppressed is null, and I get the NPE when throwableSuppressed.size is called.

The test passes if instead of using a mock in NoReachableRepositories("problem !",mock(Exception::class.java)) , I use NoReachableRepositories("problem !",Exception())

I realize it's probably better to use a real Exception rather than a mock, so my problem is solved in a way (after spending 2 hours on this..).

However, I am curious : what could cause this issue after upgrading to Spring Boot 2.6.6 which should be a a minor change ?