OK, I was able to reproduce the error. Seems like ...

# atrium

[robstoll]

OK, I was able to reproduce the error. Seems like spek is a using the wrong classpath and is not resolving all transitive runtimeOnly dependencies (maybe this is a bug of Android Studio), you can see it on top of the output when running spek (click on Test Results)

Testing started at 21:15 ...
/android-studio/jre/bin/java ... -classpath "..."

classpath is missing atrium-core-robstoll-0.12.0.jar

The good news is, I have a workaround:

// assertion
    testImplementation "ch.tutteli.atrium:atrium-infix-en_GB:0.12.0"
    testImplementation "ch.tutteli.atrium:atrium-core-robstoll:0.12.0"
    testImplementation "ch.tutteli.atrium:atrium-core-robstoll-lib:0.12.0"
    testImplementation "ch.tutteli.atrium:atrium-domain-robstoll:0.12.0"
    testImplementation "ch.tutteli.atrium:atrium-domain-robstoll-lib:0.12.0"

This way it works and FlowNavigatorTest fails with:

expected that subject: false
◆ equals: true

ch.tutteli.atrium.reporting.AtriumError
	at com.fsof.flow.FlowNavigatorTest$1$1$1.invoke(FlowNavigatorTest.kt:20)
	at com.fsof.flow.FlowNavigatorTest$1$1$1.invoke(FlowNavigatorTest.kt:14)

Maybe you can pass this information to spek, might be they can fix something.
Another good news, I am currently refactoring the architecture of Atrium. I'll reduce the modular architecture a bit and move away from ServiceLoader, I guess with 0.14.0 you won't have such problems any more.