In an obscure constraint stream such as the following, the joiner may theoretically silently fail to match:

.forEach(Something.class)
.groupBy(something -> somethingElse)
.join(forEach(Something.class)
    .map(something -> somethingElse),
    Joiners.equal())

There is no reproducer for this situation, nor can there be a test. As it turns out, during beta indexing, Drools does not use the predicate, only the index. However, if Drools did the same thing as it does for alpha indexing (there is a threshold below which the predicate is used instead of the index), the bug would show up.

Therefore I will provide the fix without a test. (I only discovered the issue by going through the code manually, looking for something completely different.)