Runegraft not working: buff duration

It seems that the "buff expire 30% slower / debuff expire 30% faster" runegraft is not working properly.

The buff I tested was Inquisitor's instrument of zeal ascendancy: Fanaticism
having the runegraft still makes it last the same amount of duration (4 seconds, tested by continuously attacking), when in theory it should have lasted 5.7 seconds.

Thanks!