TooManyActualInvocations when class is prepared for test.

[Otanikotani]

What steps will reproduce the problem?
Code:

import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.testng.PowerMockTestCase;
import org.testng.annotations.Test;

import static org.mockito.Mockito.verify;

@PrepareForTest({ SimpleTest2.SimpleFoo.class })
public class SimpleTest2 extends PowerMockTestCase {

    @Test
    public void testSetFoo() {
        SimpleTest2.SimpleFoo foo = PowerMockito.spy(new SimpleTest2.SimpleFoo());

        foo.setFoo(" ");

        verify(foo).setFoo(" ");
    }

    public static class SimpleFoo {

        private String foo;

        public void setFoo(String foo) {
            this.foo = foo;
        }
    }
}

What is the expected output?
Test passes
What do you see instead?

org.mockito.exceptions.verification.TooManyActualInvocations: 
simpleFoo.setFoo(" ");
Wanted 1 time:
-> at fr.generix.metier.bc4j.produit.SimpleTest2.testSetFoo(SimpleTest2.java:18)
But was 3 times. Undesired invocation:
-> at fr.generix.metier.bc4j.produit.SimpleTest2.testSetFoo(SimpleTest2.java:16)

    at fr.generix.metier.bc4j.produit.SimpleTest2.testSetFoo(SimpleTest2.java:18)

What version of the product are you using?
PowerMock 1.6.2
TestNG 6.9.4
On what operating system?
Win 10

Please provide any additional information below.
If

@PrepareForTest({ SimpleTest2.SimpleFoo.class })

is commented out - test passes.

if

SimpleFoo foo = Mockito.spy(new SimpleTest2.SimpleFoo());

is used instead of PowerMockito's one - test passes.

Most annoyingly, if

@PrepareForTest({ SimpleTest2.SimpleFoo.class })

is not on the current test suite class but on any other, then the test either passes or not. I guess it depends on order of tests execution. Still, it adds unpredicatability to tests results which is very annoying.