Python API GroupLayerNode content_effects() method always returns empty list

import substance_painter as sp
current_stack = sp.textureset.get_active_stack()

selected_layers = sp.layerstack.get_selected_nodes(current_stack)

for layer in selected_layers:

    if layer.get_type() == sp.layerstack.NodeType.GroupLayer:

        # this always returns an empty list, regardless of the number of effects on the group layer

        layer_effects = layer.content_effects()