The scenario is easily described. Your tank is going toe to toe with the big bad dragon, and you send your melee in. After a few seconds, the dragon realizes these melee are much better targets than the tank, and you wonder why you don't just play mass tanks if it's going to be like this.
Well, I don't. I play mass tanks. I enjoy mass tanks, but I certainly wouldn't mind some variety.
I like WoW. I like the tank having aggro while there is a selection of abilities mobs will use to threaten the rest of the group. I like that the mobs most definitely don't repeatedly go for the squishiest target with no one being able to change their minds.
Yet I couldn't make this happen in PoE1 and I can't make it happen in Deadfire. There's no aggro. Mobs might care who does the most damage (it's even available to player AI target choosing), but that really shouldn't be the tank. Mobs might care about engagement, mobs might eat a fat disengagement attack if they don't, mobs might care about distance, but none of that helps melee.
By all means, applaud the AI for picking the best target; I will not argue that it "makes sense", but I will argue it's not fun to play melee characters when I always end up wishing they were tanks, because dragons just casually switch targets and laugh at my tank. How do you stand it?