I'm creating a layout in ArcPro (v. 2.0.1) that included data from a hosted feature layer with 20 unique symbols. However, in the map extent for the layout only 4 or 5 are visible. To make the legend less cluttered I have selected the 'Only show features visible in the map extent' option for the layer in the legend properties. This results in the layer not being displayed in the Legend at all!
I've seen where other people have reported similar problems but no apparent resolution was reported.