I also just had this bug, but it was with Zephyrs funnel clouds. Not sure if its the augments fault or its just Zephyr tornadoes.
I fixed this by going down and respawning, and later i found out that running through the mini tornadoes again fixes it too.
Couldn't find anything on the wiki about Vaubans abilities and VIPs, but on the page about "Tornado" ability, it mentions that enemies with overguard and necramechs are slowed by tornadoes as if they were affected by a cold status.
Also the Zephyr was the host, but I'm not sure if that changes anything, since in the video you're the host.
I wonder whats the reason behind this bug, could it be that stalker is considered as an enemy by some abilities? It's perhaps the stalkers player model being considered as an enemy, because my drifter/operator was not effected by the slowing effect.
