Easy to get stress fractures from straight bar curling, and later on, even if your bones have somehow adapted to this, you'll likely end up with pain in the elbow region from tight/messy forearm muscles/tendons and some other stuff...
Some people's individual structure allows them to do flat bar curls and such totally pain-free, their wrists can be supinated that far under load without trouble and their forearms are more in line with their upper arms and shoulders when the arm is held in the bottom position of a straight bar curl.
A great many people's arms are structured in a way that places their hands/forearms out of line with their upper arms and shoulders in that same position, and the degree of supination necessary for a straight bar curl thus hurts them. As the OP said, the pain/irritation mostly happens in the top parts of ROM, Yates rows and such can often still be done pain-free.
I decided not to take chances and switched to alt. offset curls... Never looked back.