Can you share some screenshots showing how you configured this? Your condition will have to be something other than just 'Status <> Acknowledged', because by the time your event fires, the status may have moved on to something like 'In Progress' which may be perfectly valid. To check whether the ticket has, at some time in the past but prior to the SLA threshold, had the status 'Acknowledged', requires a different approach. I would be inclined to use one of the 'spare' fields (flag1, for example) and in the 'Update Status' activity set that flag if the status becomes 'Acknowledged'. Then test 'flag1' in your SLA event. Or, as cdtj suggests, find and cancel the SLA event when the status becomes Acknowledged. I can share some code to do that, but that's a task for tomorrow.
Hope that helps...
Regards,
James