one of the bug fixes coming with tu12 is that the memento trophies will no longer reset the cooldown with every trophy you pick up after its full, like it did in the pts. this was a welcome change which made the memento a lot more fun to use and rewarding, turns out it was a bug. making the stacks go away after you fill the buff punishes you for filling it. so my suggestion is to make the stacks decay if you don't continue picking up trophies similar to how the capacitor and bighorn stacks decay.

edit: now that ive actually played the new update the memento to doesn't seem to be acting the way i thought it was going to. i must have misunderstood. it looks like they actually added some auto pickups from a distance or something.