using a mixed grip on your deadlift raises the risk of a bicep tear on the outward facing arm.
whenever you’re dealing with heavy weight, especially heavy singles on a deadlift the supinated arm is always put in a much more strenuous position during the lift and during the lockout. this is because when the arm is turned in such an outward facing position the arm takes on a slight “curl” position and thus the bicep and the tendons in the arm have increased strain placed on it during the lift, wild swinging loose form curls also cause bicep tears for the same reason. the muscle is placed in such a tight lockout position that isn’t completely straight that when the weight becomes heavy enough you’re bound to injure it.
stick with a double overhand grip and if you can use a hookgrip or maybe even straps (if grip is an issue with you and you’re not using super light weights.). if you must use an mixed grip make sure than when you first grab at the bar, flex your tricep on your outward facing arm to full lockout to ensure that your arm is as straight as possible.