ArcGIS 10.3.1, WAB 1.3
I added an additional symbol class to a layer and republished the service (WFS) and the new symbol class does not show in the layer in an existing Portal map. I can add the service to a new map and it shows up fine.
I would be OK with just remove and re-adding the layer in the map but...
I am using this WFS for editing (new symbol actually shows up in the Edit tab of Portal though not WAB) and using the WMS of the same service to control labels for the WFS. Portal seems to have trouble removing layers in these circumstances. I can "Remove" and it removes from legend but still shows in the Properties so is not properly removing and will not properly add back. I encountered this previously and was able to overcome by removing the WFS and WMS but not so this time.
I am not sure how similar this is, but in ArcGIS Online, whenever you make a symbology change like the one you specified, you have to completely re-do the service (deleting the old service and publishing a new service in its place). At least that is what I have to do. And this is for hosted feature services (though, I have seen the same behavior for map services published to a server as well). I would almost assume this is the same for Portal, unfortunately.