Sorry, this is not a bug. I am using Debian Linux with KDE desktop, and the problem was with KDE Ibus Wayland. This intercepts text input events to provide multi-language support. As soon as I turned off this feature, Vim emulation worked. I come from Obsidian, and it also uses the CodeMirror library. What confused me was that in Obsidian, Vim support works well; I thought it was a problem with the way CodeMirror was used in Zettlr until I remembered that I was using Wayland. Thank you.