Jack's Pumpkin Shield is bugged - or in other words - completely missing a feature that is written in skill description. And as far as I know, it's here since his rework few months ago.
On description we can read, that everytime enemy gets feared while ally has shield on - that ally gets extra shield based on his 3% max health. I did many different tests but results are always the same - that 3% wasn't even programmed into his skill. He doesn't even get a tiny percent of shield, this feature is completely missing.