I just had a similar issue with VS Code and Ubuntu. The solution for me was @VonC's answer.

The lesson: I had been using the Toggle Multi-Cursor Modifier command each time I wanted to do multi-cursor using control - I assumed the Toggling was merely toggling the multi-cursor feature on - instead it was toggling the functionality to be accessed with the alt key. So now I will instead use "ctrl" to be able to jump to references / etc, and "alt" if I want to use multi-cursor.