Lucene monitor function, match single document, wraps a single document into a array of documents. Hence, it always calls the function, match many documents, which builds a MultiDocumentBatch rather than a SingletonDocumentBatch. The former uses ByteBuffersDirectory while later uses MemoryIndex.
As per documentation, MemoryIndex is a high-performance single-document main memory Apache Lucene fulltext search index. Hence, Lucene monitor should use it when matching a single document.
The patch routes match single document to a SingletonDocumentBatch.