I have tested this every which way I can think of on a Windows 7 SP1 x64 system and still get the same result with Tixati 2.66 64bit (portable). To replicate this try the following:

1. Launch Tixati.
2. Give it a couple of minutes then manually instruct Windows 7 to send the computer to sleep.
3. Result: the computer goes to sleep, briefly, then immediately wakes-up again.
4. Completely shutdown Tixati. Then manually send the computer to sleep.
5. Result: the computer goes to sleep, briefly, then immediately wakes-up again.
6. Completely shutdown the computer system, wait for a minute to make sure it is completely shutdown.
7. Boot the computer system again.
8. Once the system is running again, manually instruct the computer to go to sleep.
9. Result: the computer goes to sleep, briefly, then immediately wakes-up again.

As you can see manually sending the computer to sleep after having run Tixati does not work -- it still fails to work even after a complete system shutdown and then trying to manually get the computer to sleep.

Then I found out, after a ton of experiment, that the 'solution' is to (after having run Tixati but also having completely shutdown Tixati) allow the computer to automatically (via system Power Plan settings) send the computer to sleep. Once that is done, give it a minute to make sure it stays asleep, then wake it up again. Once that is done it is possible, once again, to manually send the computer to sleep and it stays asleep until such times as the user deliberately wakes it up again.

Clearly something very wrong going on with this. For some reason Tixati is doing something that prevents the system from being manually set to sleep and whatever Tixati has done the 'setting' survives a full system cold boot. Seems to me very much like a bug.