Steps to reproduce:
1. Open project
2. In menu, press BugReport->PrintJson
3. Observe the information in the console

Expected: container json prints out data of the its items:
{"MonoBehaviour":{"m_Enabled":true,"m_EditorHideFlags":0,"m_Name":"","m_EditorClassIdentifier":"","items":[{{"MonoBehaviour":{"m_Enabled":true,"m_EditorHideFlags":0,"m_Name":"","m_EditorClassIdentifier":"","myString":"hello","myInt":1,"myFloat":1.2300000190734864}}}{{"MonoBehaviour":{"m_Enabled":true,"m_EditorHideFlags":0,"m_Name":"","m_EditorClassIdentifier":"","myString":"hello","myInt":1,"myFloat":1.2300000190734864}}}]}}
OR json prints out the not null instance ID's of its items, ie:
{"MonoBehaviour":{"m_Enabled":1,"m_EditorHideFlags":0,"m_Name":"","m_EditorClassIdentifier":"","items":[{"instanceID":-3522},{"instanceID":-3524}]}}

Actual: only the instanceID:0 is shown under items:
{"MonoBehaviour":{"m_Enabled":true,"m_EditorHideFlags":0,"m_Name":"","m_EditorClassIdentifier":"","items":[{"instanceID":0},{"instanceID":0}]}}

Reproduced on: 5.5.0f3, 5.6.0a6
Not reproduced on: 5.4.3p3
Regression introduced on: 5.5