It's not a bug, the encounter you're mentioning was initially a tie
What happened is as follows :
Your opponent's Illusionist (10) triggered its ability after losing its encounter against your Elemental (14)
They decided to activate their losing Engineer (11) ability , meaning they add +5 to their next encounter after triggering abilities.
Then the next encounter was your Kinesis Mage (2) against your opponent's Kinesis Mage (2). The encounter resulted in a tie.
After this phase, the +5 applied to your opponent's Kinesis Mage (2) , as per the effect of their Illusionist (10) activating their Engineer (11).
So the result is correct because the +5 modifier only gets added after the current encounter resolves (if there is a tie, no ability from the current encounter triggers, and you don't check the current encounter again even after the +5 modifier is placed (That's what the "after triggering abilities" mentioned on the Engineer (11) is about) , just like it happens when a card loses an encounter after abilities are resolved, "the ability of the card that now loses is not activated"))
As an added note, the 11 from deck 2 works differently from the 11 from deck 1, so it's maybe why you got confused (11 from deck 1 adds the modifier before checking abilities and effectively changes the influence that is used to check if an encounter is won or lost to determine which card activates its losing ability, probably explains your confusion)
There is an issue related to Illusionist (10) however that has yet to be fixed, but it's not related to your game (the problem with it is that when 11 Engineer loses its encounter and 10 also loses it and copies the effect from 11, somehow the +5 gets lost in the process and the newest +5 effect cancels the previous one and 10 never gets +5 on it as a result, but that's actually an issue that i hope they fix because it effectively prevents players from playing that move on BGA (in this situation it didn't apply because your opponent's 11 initially won its encounter so this never happened on your game))
