I figured it out!
There is an incompatibility with NZXT CAM, a system monitor that runs in the background. That app has a hotkey for an overlay of Ctrl-O; even when changing that hotkey to another set of keys the problem persists in Obsidian. If I quit NZXT CAM, or set the hotkey to null, then the Obsidian Quick Switcher works fine. This bug can be closed, as it appears to originate entirely from another app.