TL;DR, my Rotos AI refused to use Vitality Plunder when fighting Wythir Siphi. Not sure why.

The other day, I was fighting Wythir Siphi in classic Arena. Whenever I see a solo/duo Wythir, I swap my AoE nuker for Rotos, so he can reduce her max HP. I also know fights with Wythir are likely to take a couple of minutes, so I entered the fight and went afk.

10 minutes later, I came back and neither Wythir, nor the Siphi had any HP reductions. I watched the fight for a few turns to see what happened and Rotos was not using Vitality Plunder at all. I did not set the AI, but normally the default AI has no issue draining Wythir's HP and winning. This time, the AI refused to do so.

Does Rotos have some sort of special AI when up against Siphi? I fought a team with 2 Wythirs and 2 Nekhrets once. It took 17 minutes, but Rotos eventually killed them. Yet adding in 1 Siphi and he refused to do his job.