Oh wait... so if you never upgrade the speed on Voodoo Sapper, the damage upgrades will work fine? I always upgrade speed first just because I thought I had to. Then I never upgrade the damage again because I'm concerned it will reset it to base if I don't do the speed again first. Either way, it's disappointing that the team can't even replicate this issue. Knowing that, it seems like it's destined to be this way for a while.
!PIZZA
Yes - if you update the Speed 1st, you're fine.
If you do Spd, Atk, Atk, Spd... RESET.
If you do Atk Spd... RESET
After the first Speed bump, the second and third are not worth it.
2 Seconds -> 1.8 Seconds (-10%)
1.8 -> 1.764 s (-2%)
1.764 -> 1.729 s (-2%)
I just typically upgrade the damage, for a better "alpha strike" trying to 1-Shot enemies :P