There was an anon complaining about tanks diving the backline in Colosseum so I looked into it after I started to notice it too. It turns out Ranged units target the nearest enemy(can overlap) and Melee/Support units(Tanks) target the closest untargeted enemy (with some special exceptions). That means that if your party leader is Ranged and the enemy Tank is in the front they are the first to be targeted and your Tank will target the next unit which could be in the backline. It also depends on your party order so if you have another Melee unit ahead of your Tank they will take the next closest enemy and your Tank will go for the next(which can be even further away) and so on. That means if you manipulate the order and placement of your team you can guarantee the first moves in a match and create an easy win or eat a total defeat even in imbalanced matches.
>Example:
Here my Daisy is set up to target their Eunha because my leader Eunha/Gab has their Craig targeted first, and their Craig targets my Eunha because all their ranged units start off targeting my Daisy. Because I set my Daisy closer to their Eunha she will taunt before their Craig which means I can pull him and aggro all their units away from my backline for an easy win. Something else to mention is targeting resets after a unit uses a skill/finishes their attack string so if they are taunted that's when they will start attacking the tank. That also means Tanks will reset their initial targeting after taunting and go for the closest untargeted unit. Their Craig gets immediately aggro'd but has to do his taunt before turning around to target Daisy who is already moving to the next untargeted unit in their backline because Craig is still targeted.
TLDR: It's not bugged and I finally understand Colo targeting now.