While working on
SOLR-10583 I was getting some odd test failures that seemed to suggest we were getting false cache hits for Join queries that should have been unique.
tracing thorugh the code, the problem seems to be the way TermsQuery implements equals(Object). This class takes in the fromQuery (used to identify set of documents we "join from") and uses it in the equals calculation – but the information about the join field is never passed directly to TermsQuery and the BytesRefs that are passed in can't be compared efficiently (AFAICT), so 2 completely diff calls to JoinUtils.createJoinQuery(...) can result in Query objects that think they are equal() even when they most certainly are not.
At a brief glance, it appears that similar bugs exist in TermsIncludingScoreQuery (and possibly GlobalOrdinalsWithScoreQuery, but i didn't look into that class at all)