The schema of MapType can not be printed correctly

import spark.implicits._
case class Key(a: Int)
case class Value(b: Int)
Seq(
  (1, Map(Key(1) -> Value(2))), 
  (2, Map(Key(1) -> Value(2)))
).toDF("id", "dict").printSchema

root
|-- id: integer (nullable = false)
|-- dict: map (nullable = true)
| |-- key: struct
| |-- value: struct (valueContainsNull = true)
| | |-- a: integer (nullable = false)
| | |-- b: integer (nullable = false)

root
|-- id: integer (nullable = false)
|-- dict: map (nullable = true)
| |-- key: struct
| | |-- a: integer (nullable = false)
| |-- value: struct (valueContainsNull = true)
| | |-- b: integer (nullable = false)