In the layers widget, the "Show legend for all visible layers by default" toggle works for layers that are turned on when a map initially loads. For all subsequent layers you turn on, however, the legend still needs to be turned on manually. It would be helpful if the legend turned on and off with each layer as the layer is turned on and off, instead of just working for the initial map view.
I get the same behavior in Portal. This would be a good thing to post to the ideas forum!