I tried a bit around with the newest update today (Software Update: FP6.QREL.15.151.0 📲)
Without always on, it still seems to work reliably.
With always on, it seems only not to work if I try to wake it up directly after turning off the screen.
If I turn the screen off and wait for a few seconds, it seems to always work.
There is always the moment when the AOD switches from very dark to a bit brighter after a few seconds. If I try it before, the double tap does nothing, but after it works.
Can anyone confirm this?