Hi Carole,
I'm assuming that the "extra" legend items are coming from an ArcGIS Map Service layer. If they are not, if the data source is local like a file geodatabase, then this is a different bug. If so, please let us know because at this time I'm only aware of this issue being particular to ArcGIS Map Service layers.
To get around the bug you will need to create a new legend that does not include this layer when the legend is created. You can do this in two ways. You can create the legend first and then add the service to the map. If the service is already in the legend you can create a legend without this layer by selecting the layers you do want in your legend and then insert the legend.
From Add a legend—ArcGIS Pro | Documentation
Optionally, if you don't want all the layers added to the legend, expand the map in the Contents pane and select the layers to add to the legend. If no layers are selected, a legend is added containing all applicable layers from the map contained in the default map frame.
Did you try either of these workarounds? If you did please let me know of the steps you took. I may have missed something in the workaround and would really like to get you to a workaround that is not putting a white box over the extra items if possible.
We are currently evaluating if this something we can include in a service pack.
Thanks for the feedback,
Tom