Can you check somehow if the system was actually sleeping or not at the time?
would not know how, but 'd assume with no app active, within twelve hours no action, it should be actually sleeping very well. But then, Manjaro KDE is quite a mess… Currently Bluetooth does not work, display scaling does not work, and whatnot else 
We should be able to check for sure.
Anyone knows the way?
agreed, but then, double tap may not be the most important feature on their laundry list. I’d see a well working PIM solution for caldav/carddav as a higher priority. Plus, double tap after 12 hours seems quite ok, sleep or not sleep.
If it doesn’t sleep, it might be an issue per se.
Let’s resume some memories of what you can do with those tools:
https://m.youtube.com/shorts/69U8riSVkLI