I tried again just now but in Firefox on macOS this time. Different computer, different OS, same result.
Both of the times I created new projects (tested with and without the Keyboard plugin added).
Another somewhat related issue is that both Safari and Firefox seem to lose focus when tab is pressed, as Construct doesn't seem to capture the tab key's input. This is true even when "Keyboard: Tab is down" is specified.
I would attach a GIF that shows the behavior but the forum won't allow me to upload animated images.