This event occurred on the server. If one player attacks Luxtructosaurus and then takes off (mod items) and stands still, then Luxtructosaurus will have a target only for this player, and another player who will stand next to Luxtructosaurus and beat him, he in turn will ignore him, which completely breaks the gameplay. The Luxtructosaurus feature itself, chasing its target over a huge distance, jumping 100 blocks behind it, may be causing this.

This is due to its insane detection range of 256 blocks, and mostly likely could be fixed by turning on the gamerule UniversalAnger