MultiSearcher is using:
queries[i] = searchables[i].rewrite(original);
to rewrite query and then use combine to combine them.
But SpanRegexQuery's rewrite is different from others.
After you call it on the same query, it always return the same rewritten queries.
As a result, only search on the first IndexSearcher work. All others are using the first IndexSearcher's rewrite queries.
So many terms are missing and return unexpected result.