Just use Rafflesia -- ARKit doesn't seem to like the Unity logo

Assets/Scenes/ImageTracking/Images/ReferenceImageLibrary.asset

     m_SpecifySize: 1
     m_Name: Rafflesia
     m_Texture: {fileID: 2800000, guid: 8185ee60df6f2a44e8c0cf22387f31aa, type: 3}
-  - m_SerializedGuid:
-      m_GuidLow: 5023543680583928437
-      m_GuidHigh: 16852023376600186500
-    m_SerializedTextureGuid:
-      m_GuidLow: 5833551047903198129
-      m_GuidHigh: 6063111697491653035
-    m_Size: {x: 0.1, y: 0.1}
-    m_SpecifySize: 1
-    m_Name: Unity Logo
-    m_Texture: {fileID: 2800000, guid: 4560bbb1efc550f4ab718112f77f2454, type: 3}