Lambda not testable with Spock?

I've got a problem with testing Java code using Groovy/Spock. I wrote a short lambda-expression for filtering a list:

public List<myVO> getMeSomething() {    
    return RequestUtils.getRequestAttribut("somestuff", () -> getList().stream().filter(mt -> 
    !mt.isArchived()).filter(mt -> !mt.isSomething()).collect(toList()));
}

I used this to test it:

 when:
 2 * contextData.getList() >> [item1, item2]
 List<myVO> list = context.getMeSomething()

 then:
 list.contains(item1)
 !list.contains(item2)

This worked when run as single test. When I executed all tests in the project this one failed. I tried to extract the "getList()" into a local variable but it had no effect.

Then I changed the method to:

public List<myVO> getMeSomething() {
    List<myVO> result = new ArrayList<>();
    for (myVO vo : Objects.requireNonNull(getList())) {
        if (!vo.isSomething() && !vo.isArchived()) {
            result.add(vo);
        }
    }
    return result;
}

And suddenly the test worked where it failed before. Is there any reason for this behavior? Is there a problem with testing a lambda with Spock?

Thanks!