The Layer symbology fails to update in the Script tool if the layer is accessed through a Script tool parameter.

Workaround

If layer is accessed through:

p = arcpy.mp.ArcGISProject("CURRENT")

m = p.listMaps('Map')[0]

layer = arcpy.GetParameter(0) # FeatureLayer Parameter

fld1 = arcpy.GetParameter(1) # String Parameter 

fld2 = arcpy.GetParameter(2) # String Parameter

sym = layer.symbology

layer = m.listLayers(layer.name)[0]

There is no issue.

Use stand alone Python script using arcpy.mp.ArcGISProject.SaveACopy().

Steps to Reproduce