iTunes 12.3 seems to have a bug regarding some of its menus. Previously, if you clicked the … next to a track (that button only appears when you hover your cursor over the track), you’d see a popup menu that included an option to create a Genius playlist. Now, in iTunes 12.3, it only lets you Start Genius, which starts playing a selection of tracks, picked by the Genius feature, but as it does so, it bounces around your library.
You can still create a Genius playlist, which gets added to your playlists sidebar, and which you can edit manually. To do so, right-click on a track and choose Create Genius Playlist.
I don’t know why Apple removed this option from the first menu; I assume it’s a bug. Note that there are lots of other options in the second menu compared to the first.
I still don’t understand why Apple thinks two types of contextual menus in iTunes are a good idea, but that’s another story…