Hi I’m here to ask for some clarification about a behaviour that I encountered while trying to index matrices with RemoveBlankFieldUpdateProcessorFactory in the update processor chain, expecting an error since this syntax isn't supported in Solr. Here's an example: if I try to index a multivalued field with values [["cat", "dog"], ["lemon", "orange"]], the field is modified to ["cat", "dog", "lemon", "orange"]—flattening the 2x2 "matrix" into a 4-item vector. This flattening seems to happen in the mutate function <https://github.com/SeaseLtd/solr/blob/c99af207c761ec34812ef1cc3054eb2804b7448b/solr/core/src/java/org/apache/solr/update/processor/FieldValueMutatingUpdateProcessor.java#L60> of FieldValueMutatingUpdateProcessor.
This allows the field to be populated even though the syntax is incorrect. Is there a reason for this behaviour? Thanks, Nicolò
