The output product from the Write operator (within the graph) obtains a reference towards the GRM operator output band before actually running the GRM operator.
The GRM operator output band changes its source after running the algorithm, but the output product within the Write operator remains with the old reference towards the source band, created in the initialisation step.

See: https://forum.step.esa.int/t/snappy-genericregionmergingop/25113