Soul Ripper not giving the correct amount of souls

I am using a min rolled Soul Ripper with 20 maximum charges for vaal lightning strike.

At the start of my maps (or any new areas), I noticed that Soul Ripper gives me less than 20 souls when used (ranging from 8 to 14 souls). This only happens the first couple uses and then it behaves as it should, giving 20 souls per use.

I gain 20 flask charges every 3 seconds and made sure I didn't have any "reduced flask charges used" applied to my flasks. I would like to know if this is a bug or if there is a hidden reduction in souls gained at the start of areas (or something else) that I am missing.

Thank you