Soul orbs dropped from troopers behave inconsistently to light melees. Sometimes you will have the hit sound but the orb won't break, while other times it will be secured.
Soul orbs are claimable only after a very short delay (maybe about 0.1s) after it spawns/forms. This bug happens when you hit a soul orb after it spawned but before it is claimable. You can even reproduce the bug with guns. It is just much more noticeable with melee hits because of the sfx and vfx.