N1QL query works even with missing predicate created using couchbase secondary index

Question

I have created a secondary index in couchbase:

CREATE INDEX `data` ON `bucket`(`field1`,`field2`,`field3`, lower(`field4`)) WHERE (`field5` = "CONSTANT")

When I execute this query on couchbase:

select document.* from bucket AS document WHERE document.field5 = "CONSTANT" 
AND document.field1 =  "docId"
and (document.field3 like "%" or 
            lower(document.field4) like "%");

Even without giving the value for one of the leading predicates(field2), the query still seems to work properly. Can anyone explain why?

This is the query plan that is returned when adding "EXPLAIN" in front of the query:

    "#operator": "Sequence",
    "~children": [
      {
        "#operator": "Sequence",
        "~children": [
          {
            "#operator": "IndexScan3",
            "as": "document",
            "index": "data",
            "index_id": "6588bcds21b213",
            "index_projection": {
              "primary_key": true
            },
            "keyspace": "bucket",
            "namespace": "default",
            "spans": [
              {
                "exact": true,
                "range": [
                  {
                    "high": "\"docId\"",
                    "inclusion": 3,
                    "low": "\"docId\""
                  }
                ]
              }
            ],
            "using": "gsi"
          },
          {
            "#operator": "Fetch",
            "as": "document",
            "keyspace": "bucket",
            "namespace": "default"
          },
          {
            "#operator": "Parallel",
            "~child": {
              "#operator": "Sequence",
              "~children": [
                {
                  "#operator": "Filter",
                  "condition": "((((`document`.`field5`) = \"CONSTANT\") and ((`document`.`field1`) = \"docId\")) and (((`document`.`field3`) like \"%\"]) or (lower((`document`.`field4`)) like \"%\")))"
                },
                {
                  "#operator": "Project",
                  "result_terms": [
                    {
                      "expr": "`document`",
                      "star": true
                    }
                  ]
                }
              ]
            }
          }
        ]
      }