I have noticed this too, and I believe it’s related to Ubastet’s fix… Infernus has always selected its two targets at the same time: in this particular example, he selected 4th and 3rd troops. Before 4.0 he would have cast first on 4th, killing the 3rd troop with splash damage, and then cast on the (now empty) 3rd spot, with the resulting splash damage killing the 2nd troop.
What I think it does now is cast on the 4th (killing the 3rd too), then check whether its second target is still alive and only if it’s alive do the second cast. This was likely introduced to prevent Ubastet’s double kill, but it also resulted in yet another small nerf to infernus and any other troop that works like this.
I agree with you that the target selection should be changed to be sequential (choose first target, cast, then choose the second from the remaining troops and cast again), not only for Infernus but for other troops with the same mechanic that sometimes lose damage output for no good reason (e.g. ship cannon), hopefully the devs will agree!