Advanced BBcode Box sounded like a good guess, but it shows up for me with or without that, even when using Win10 style. Circled in red in both screen shots.
Assuming the event template Goldy pointed to is firing, the only thing display of the icon is conditional on is the newly-implemented permissions. So if you were playing with the permission aspect, maybe that's what preventing it from displaying.
Edit: Just realized your screen shot is showing the textual version of the s_username button, too. An additional guess I would throw at that is that you do have "display on posting page" enabled in the custom BBCode settings, but do not have the extension-specific permissions set to allow your user to use the BBCode. That's how I would get the display shown in your screen shot, for what it's worth. From what I can see, the extension expects you can just leave "display on posting page" disabled in the custom BBCode configuration, and the extension is going to display the button (if you have permission) regardless.