Crucial: The issue is NOT related to `#if UNITY_EDITOR` and is fundamental issue relating to ScriptableObject runtime serialisation as of 2023.3.0a11 trunk, the error message is potentially lead to this assumption therefore -the summary needs updating in due course.- Done

Steps to reproduce:
1. Before opening the project, open the "log.txt" file attached and note the errors related to serialization and #ifdef UNITY_EDITOR
2. Open the attached user's project "SerializationErrorRepro.zip"
3. Build the Standalone Player and run it
4. Check the player the log file and note that there are no serialization errors
5. In the project's "Scripts/Runtime/" folder, delete the Assembly Definition file named "Game"
6. Add a new Assembly Definition file in the project's Scripts/Runtime/ folder and name it whatever you want (for example, "Game1")
7. Once the project compiles, build the Standalone Player
8. Run the build, and in the Player log file, note that the serialization errors are occurring

Expected results: ScriptableObject serialization for in-editor-only field does not break in build when adding assembly definitions
Actual results: ScriptableObject serialization for in-editor-only field breaks in build when adding assembly definitions

Reproducible with: 2021.3.30f1, 2022.3.9f1, 2023.1.12f1, 2023.2.0b9, 2023.3.0a5

Reproducible on: Windows 11
Not reproducible on: No other environment tested

Notes:
-If a subclass of ScriptableObject is moved into or to another assembly definition after the Unity Library folder has been established, the serialization of instances of that ScriptableObject will break in a build if they contain editor-only fields (serialized fields wrapped by #if UNITY_EDITOR).