ArcGIS Pro 2.2.4
Repo steps below.
- Add a feature class to map, symbolize by unique values.
- In the "Field 1" dropdown, set a custom expression using Arcade. I simply concatenated 2 text fields using ", ". I dont' think the expression matters though, just so your using it
- Add map to a layout, set up a simple map series
- Expand the legend item in the layout, select the feature class where you set the unique value symbology expression.
- In the Element Pane, check "only show features visible in the map extent". In the dropdown under that you can select either option "within map frame" or "within map series".
- The legend does not update, all features in the above feature class are shown, (not filtered by map frame extent) this is the issue.
- If you remove the custom expression from the symbology in #2 above, everything works as expected