Handlebar slips in quill stem clamp when going over bumps
A few things to think about:

Are you using grease or threadlocker on the bolt threads? This will minimize corrosion and galling problems, and means that the same final torque value is actually clamping tighter than if the threads are not lubricated. Personally, I would use grease on this bolt, but others may differ. Ask Kalloy whether their torque spec is using lube or not.
Does the handlebar looseness change over time? Is it the same &quot;looseness&quot; from the start, or does it get looser the longer you ride? (Is the bolt loosening during the ride?)
Does the handlebar indeed measure 25.4mm in diameter at the clamping point (measure using calipers)?
Are the threads on the bolt and in the stem's clamping region in good shape?
Is the bolt used and might have been stretched from being overtorqued in the past?
How do you know your torque wrench is correctly calibrated? You might try a different one, or a visually-calibrated one like a beam-type if that's not what you're using.

fwiw, found this Kalloy/Uno stem online and by zooming in on the photo for the 80mm model it reads &quot;15Nm&quot; next to its clamp bolt. Not sure if it is the same model as yours but it looks very similar:
https://www.performancebike.com/kalloy-road-quill-stem-black-25.4mm-80mm-13-kl-80-bk/p863539