I does work all the time when the phone has locked recently. Is it possible that there are two sleep states, with a deep sleep that double tap wouldn't wake? Just throwing hypotheses, but we will need more tests to check when double tap works or does not work.

Not yet Fellfrosch, I haven't even had the time to look into it yet. However I got qwerty fully working including / and ? using the Alt key thanks to TheKit's help (see Github). To explain to those who haven't tried SFOS on the Pro1, the problem with these symbols is they require using the yellow arrow modifier. However, the keyboard on SFOS behaves like a desktop keyboard so far, meaning the yellow characters (second layer symbol) are activated with Shift. This works for special character keys and numbers, but alpha keys just turn into capital letters when used in combination with Shift, as expected. And the issue is ? and / are the second layer of alpha keys, so Shift doesn't trigger them. Therefore, TheKit proposed a small modification to enable them using Alt. Many more possibilities should be possible once we figure out how to use the yellow arrow modifier in SFOS.