Hi, 

Doubtful it's a bug. likely a combo of set bonuses, lore of steel masteries and such coming into play. could you show builds and masteries? 

I would also like to point out to the OP, the 2 champs you have are closely matching in speed. While it looks like on paper that Serris "should" go first, there's also a random generated speed bonus at the beginning/end of their turn.

For example, I have an Arbiter with 380 speed and a Mezonel with 228 speed. So, my Arbiter goes first and increases the turn meter 30%. However, about a third of the time, my Arbiter gets her second round before Mezonel gets her first turn. There's no gear switching or a decrease speed debuff the opponite uses, there's just a random element that makes Arbiter twice as fast as Mezonel does. It's the same reason you can crit a champ for 60,821 and using the same champ vs. champ, you can do 68,044. There's some randomness to the numbers that are not static.