Sprite relying on Alpha not looking good when turned into flipbook

Hi everyone, I’m working on a 2D prototype (in a 3D world) and desperately trying to create a nice shadow from a flipbook composed of alpha sprites.

339431-shadow-issue.gif

It’s looking as I want in the Content Browser but as soon as I check the flipbook. It’s just a round shape with no gradient towards alpha and I have no idea why. Anyone having a clue on this?

339432-shadow-issue02.gif

Thanks in advance for your help!