Unsure if this is a bug or intended as this behavior isn’t mentioned in the tooltip. Regardless of how many minimum/maximum aftershocks you have for earthquake it will only ever spawn a single aftershock per enemy available. If you only have a single enemy nearby (e.g. a boss or high hp rare) you only get one aftershock instead of your minimum amount (up to 5 minimum with investment). I hope this isn’t the intended behavior.
Second note, it seems there was some similar reports in the past, but the Upheaval node appears to have no effect on the aftershock behavior or damage.
Thanks for updating the tooltip. I had hoped to make a aftershock build stacking the DoT of upheaval on big bosses, but I guess that’s not going to be at thing.