Just finished the Lagon fight, playing as a beastmaster with summons. I can’t reproduce on this character, and couldn’t see a previous bug report for this.
When playing a beastmaster and fightin Lagon, using your leap from the middle of the fight area towards Lagon with a Sabertooth summoned will cause the sabertooth to clip outside the arena, and according to the map land on the other side of Lagon.
When Lagon uses his sweeping breath attack, they will use it to turn towards the sabertooth, turning around to face away from the player. They will stop attacking your creatures, and if your saber has enough HP/regen will not have enough DPS to kill it, and will stop attacking the player character at all, making the fight cheesable.
Using leap will cause the saber to fly back to the player character, and Lagon will return to attack you as normal, however I was able to reproduce the leap bug several times throughout the fight.