When layers are used as inputs for the ArcPy Merge tool, the MERGE_SRC field is populated with a globally unique identifier (GUID) instead of the layer's name.

Solución alternativa

When calling the name property of the layers class in the inputs of the Merge tool, the MERGE_SRC field can reference the name successfully, but this only works in ArcGIS Pro and does not work in a standalone script.

Pasos para reproducir