Raging Cry Seismic Cry Rage Generation

Something appears to be off with the rage generation of this support, but only with seismic cry. This appears to work correctly with infernal cry.

Most warcries, regardless of # or power of enemies in radius appear to generate no rage, a low percent of the time rage is generated.

Reproduction:
equip to seismic cry, stand next to a unique enemy, continuously warcry.

Expected result of test:
rage is progressively generated up to the maximum amount by each individual warcry

Actual result:
rage spontaneously jumps from 0 to maximum after some time, rage can be observed to decay due to lack of generating source even during continuous warcry usage.

+1 also noticed this.

Afaik it works only if you actually stunned a boss.
In other words, for the support to generate rage, warcry has not only to be used, but also to exert attacks. Since Seismic exerts attacks only when a boss is stunned, it grants rage aswell only when a boss is stunned. This is how I understood that interaction, may be wrong.
How strong could Seismic be as a rage generation warcry, since it has no cd, right? I think this is exactly the reason why it doesn't work like that.