Hi Frido,
Yes, the Keyswitches in StudioOne are not working by default. StudioOne hijacks the Keyswitches that are sent by the Synchron Player for it's internal Articulation Switching system. When you open the Sound Variations window, you can Switch the Articulations there by clicking with the mouse (or inside the Synchron player as well), or inside a recorded MIDI event, you can of course assign the Articulations (the intended use case I would assume).
However, when you change the Sound Variation "mode" from "Enable Keyswitches" to "Use Activation Sequence", you can use your Keyboard to Keyswitch as you normally would do when you Switch while playing. The ability to assign the Articulations inside the Sound Variation Lane then is still possible, so "best of both worlds". You find this setting when you click the wrench in the Sound Variations lane of the MIDI Editor. Then you see allthe Switches laid out - on top you see the name of the Patch, and right next to it to the right you should see "Enable Keyswitches" - click this dropdown and select "Use Activation Sequence" (See attached Screenshot)
Short explanation on the "intended use case I assume": You would input notes with a long patch, get the stuff in, riding faders or whatever. Then you would edit that MIDI sequence and shorten/lengthen note, move them, assign articulation to shape the performance. That's how I currently mainly work as well. As said in the second paragraph - change to "Use Activation Sequence" and you can use the Keyswitches while playing (or to select beforehand via MIDI-Keyboard).
I hope that helped,
Best, Mat