Until 17.10 I mostly used Unity and never had this issue (I had GNOME installed and seldom used it). But since the upgrade I switched to GNOME, where the shortcut works and I find it better because it cycles through sources in the recently used order.
After a few months I decided to reinstall Ubuntu because it was too laggy, so the Unity session I have now was installed manually later. Then I noticed that it was GNOME especially that was too laggy on my 4GB RAM laptop, so I moved back to Unity which is much more fluid.
Here comes the issue, pressing Super+Space doesn't switch the input sources, and the weirdest thing is that Shift+Super+Space shows and horizontal switcher for English and French, instead of the vertical drop-down switcher from the panel where I don't even have an English source (I have French, Japanese and Belgian sources).
Note : I noticed that other shortcuts that use the Super key don't work as well as the other shortcuts : i.e. Super+E which I set to open nautilus as root only works when I press Super then E then release E and finally release Super. I don't remember having a 'release order' before, could it be related to the issue ?
Note 2 : I know I can bind another shortcut for this task (and it works), but I find the default one pretty good and I don't like this Super key malfunction.
About the input sources showed when I do Shift+Super+Space (English and French), I found that they are the same as in Fcitx Configuration (which key binding is set to Ctrl+Space) and as in IBus Preferences (which key binding was also set to Super+Space, but I set it to something else to test, however Super+Space still does not work).
gsettings get org.gnome.desktop.wm.keybindings switch-input-sourceoutputs.
gsettings get org.gnome.desktop.input-sources sourcesoutputs.
['<Super>space'](as expected I suppose)
gnome-tweak-tooland use Tweak to set an alternative shortcut for switching input sources.