attached is a simple test, it adds a single document "foo bar" to one index,
and another document "foo baz" to another.
if you do the query "+foo -ba*", the multisearcher rewrites this to:
(+field:foo -field:baz) (+field:foo -field:bar)
This causes both documents to match the query, when really neither should.
instead the query should be (+field:foo -field:baz -field:bar)
if you run the test with -Dtests.verbose=true you can see the rewritten form.
the reason this only appeared with a certain document count for the issue on the
user's list is because they were using CONSTANT_SCORE_AUTO and with that
document count it was deciding to use a constant-score boolean rewrite method.