One more thing to consideer.
How were you tightening the stem bolts?
If doing up the faceplate on a four bolt stem, you need to bring all four bolts up to torque together, do not tighten one then the other, then the other etc.
If the faceplate is wonky and you tighten one bolt, then another, the first bolt head can be put under a lot more strain.
If you were to just crank up the bolts to 8nm one at a time, i am not surprised one snapped.
If you smoothly tightened them, in a pattern that means you tighten; bottom left, top right, bottom right, top left. Moving diaganally across the bar, and repeat, getting slightly tighter and tighter each time, then I bet you can achieve the recommended torque easily without snapping anything.
And also remember to grease the threads, so that they turn smoothly and don't bind.
Personally, i would use a t-bar allen key to spin the bolts up, then bring the bolts up to torque as above, probably tightening each bolt seven or eight times by feel. Then check with the torque wrench. By doing it this way i am 100% confident that the stem faceplate i tightened correctly.
And 12nm on a stem seems very very tight. Most modern stems using an M4 bolt are 5nm, the Sunline is 6nm. A fourbolt faceplate holds that bars very tightly and shouldn't need excess bolt tension to hold it.