Reproduction steps:
1. Extract the attached “IN-107076_TestFiles.zip“ archive
2. Open it inside the terminal
3. Locate and copy the path of the UnityYAMLMerge executable inside your Unity Engine folder
4. Execute the “{path to UnityYAMLMerge executable} merge -p Base/YamlPrefabVariant.prefab Left/YamlPrefabVariant.prefab Right/YamlPrefabVariant.prefab Merged/YamlPrefabVariant.prefab“ (the command might need some minor modifications depending on the platform)
5. Open the “Merged/YamlPrefabVariant.prefab“ inside a text editor
6. Observe

Expected result: The UnityYAMLMerge falls back to an interactive merge tool
Actual result: UnityYAMLMerge merges and keeps both “m_StringField” entries

Reproducible with: 2022.3.64f1, 6000.0.53f1, 6000.1.11f1, 6000.2.0b9, 6000.3.0a1

Reproducible on: M1 Max MacOS 15.5 (Tested by CQA), Windows 11 (User’s platform)
Not reproducible on: No other environment tested