Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. Open Package Manager and install Cinemachine package
3. In Hierarchy window create Cinemachine Camera (Cinemachine > Cinemachine Camera)
4. Go to Inspector > Cinemachine Camera > Add Extension > select “CinemachineFreeLookModifier”
5. In Modifiers list press “+” and select “Tilt”
6. Observe the Console window
Actual results: “The type Unity.Cinemachine TiltModifier is being serialized by [SerializeReference], but is missing the [Serializable] attribute. To suppress this warning add [Serializable] or use [MakeSerializable].” warnings it thrown in the Console window
Expected results: No warnings are thrown in the Console window
Reproducible with versions: 6000.3.0a5 (36d7e9fd6645), 6000.3.20f1, 6000.5.4f1, 6000.6.0b4, 6000.7.0a2
Not reproducible with versions: 6000.0.78f1, 6000.3.0a4 (53ab01ef978b)
Tested on (OS): macOS Tahoe 26.5.1 (M1)