This set is activated by building up 5 Bone Stacks with Light Attacks on a 1s cooldown, and there is 3s downtime between a proc and building stacks again. With the 1s GCD on skills, this should mean it procs every 8s when Light Attack weaving between skills. In practice this is not occurring, and the time between Kjalnar procs can vary randomly based on millisecond timing variations between Light Attacks.
Here's an example:

You can see that there was over 10s between the Kjalnar's Nightmare procs, and 10 Light Attacks during this time. However the expected 8s cooldown was not correct. This seems to be caused by the time between the Light Attacks at 17.909 and 18.887 (only 0.978s apart so the 2nd one did not generate a bone stack). This occurred again at 20.996 and 21.975 (only 0.979s apart).
Technically the set does appear to be following its own 1s cooldown on generating bone stacks, however with the 1s global cooldown on all skills, it should not be possible to skip stacks when Light Attack weaving (I was using Force Pulse between each of these Light Attacks during the test). This could be caused by slight variations in latency. The easiest way to fix this would be to make the set have an actual cooldown of around 0.9s for building bone stacks on a Light Attack, so that when it is off the 3s cooldown, each Light Attack will add one bone stack regardless of millisecond variations in timing.
As a related note, the updated Ice Furnace set is having this same issue. Frost damage that is "1s" apart misses the cooldown period half the time, resulting in about 33% fewer Frostfire procs than expected.