How to reproduce:
1. Create and open a new project
2. Install the “Multiplayer Play Mode” package
3. Go to Window > Multiplayer > Play Mode Scenarios and create a new Scenario
4. Select the new Scenario in the Toolbar and Enter Play Mode
5. Observe the Console

Actual result: No warnings are thrown
Expected result: Warnings are thrown: “The type UnityEditor.MultiplayerModule Unity.Multiplayer.PlayMode.Editor.EditorMultiplayerPlaymodeDeployNode is being serialized by [SerializeReference], but is missing the [Serializable] attribute”

Reproducible with: 2.0.0-pre.1 (6000.3.0a5), 2.0.1 (6000.3.2f1)
Not reproducible with: 1.6.2 (6000.3.0a4), 2.0.1 (6000.4.0b1, 6000.5.0a3)

Reproducible on: macOS Tahoe 26.1, Windows 10 (by user)
Not reproducible on: no other environments tested

Note: After triggering the warning with the custom Scenario, switching back to Default and entering Play mode still throws the warnings