Handlebar slips in quill stem clamp when going over bumps

I bought this new Kalloy quill stem, with Sakae 25.4mm handlebars as pictured below. The handlebars were purchased at an eBay auction.

Here’s the setup from the side

I’ve tightened the handlebar clamping bolt (m8 size, 10mm thread engagement) to the maximum torque of 10 N m as printed on the stem, photo here, which does fine when riding on flat surfaces, but as soon as i hit a bump when on the hoods, the handlebars rotate forward. Adding fibre grip doesn’t solve this problem.

When looking up the torque values of typical quill stems they range between 14 – 30 N m, Park tool’s recommended torque ranges reflect this.

Would it be okay for me to torque higher than 10 N m despite what it says on the stem? As far as I know, Kalloy is a fairly reputable company when it comes to components, so I’m surprised the torque is so much lower, and that this slippage problem is occurring.

My current strategy is to incrementally up the torque until it’s at an acceptable level. If that still doesn’t work, I’m considering using a shim.

Please let me know if there’s anything I should consider!

EDIT:

I’ve heard back from the store I bought it from and this was their reply:

Good morning, The first thing I would check is the handlebar diameter
just to make sure it is within spec. We see a lot of bars that are on
the low side of the specification, 25.2mm is probably ok, anything
below could be an issue. If it is within spec you should be able to
safely exceed the recommended torque, just make sure the bolt threads
are greased. I would have sold over a thousand of these stems over
the past 10 years or so and more than likely the majority of customers
wouldn’t bother with a torque wrench, no doubt they have taken them
well past 10 N m I would go up in 0.5 N m increments

I rechecked the diameter of the handlebar, and they are within spec. Bolts are also greased.
I’ve tested it up to 15 N m (as far as my torque wrench allows), and will try shimming it with a beer can before borrowing a torque wrench with higher torque setting. So far seems promising!