I've seen this behavior too. IIRC the stale cleaning was not instant, it took some time to be applied.
Regards.
Gianluca
Confirmed.
Quite soon after the command I saw that the status of the "Current" snapshot line changed from Locked (it was so since 8/11) to OK, but the task remained at least for half an hour.
Now, after about one hour and a half I connected again to the web admin gui and I see 0 Tasks, so the problem has been resolved.