People are reporting another potential bug with Malygos - arcane breath hitting people who are standing behind him without a change in direction. One theory posits that pets are getting hit by the breath and living through it, but bringing the debuff to other members of the raid as they reposition and the debuff ticks off.
World of Warcraft - English (NA) Forums -> [BUG] Malygos Arcane Breath
Someone posted a video in that thread that would account for this behavior, and if you'll accept my anecdotal evidence I can say with certainty that it fits the behavior of what my guild experienced last night. Several attempts saw very strange behavior with Malygos basically doing an "arcane fart" and hitting people who were clearly behind him (including ranged people) and this was even acknowledged by the officers. We did an attempt with no pets after someone found a similar bug report, and this behavior disappeared completely.
I'm not sure if this would be considered a bug with pet pathing, a bug with pets getting the debuff in general, or if it's merely a pathing issue that we need to adjust to. I tend to think pets should just not get the debuff, the same way they can't trigger flame wreath on Aran, etc. After this patch, I noticed that my pet despawns during a vortex and respawns immediately after. As long as he's on passive (or defensive and I do not attack) there appears to be negligible hesitation in him running away from Malygos with me, so I find it unlikely that he is the source of these breath debuffs. I am not sure how death knight pets behave though, and I know that they received some changes this patch.
If anyone has any ideas on how to test this further - whether or not it's a bug or a pathing issue we need to solve - that would be great.