Yeah that was always the fear and I'd try to get the slack out of my under grip arm when I do go mixed.
I also noticed my under-grip arm side my lower back and sometimes my knee on that side would sometimes hurt. I did like 95% of my max and my back and knee feels normal, no belt either.
If you can't get the slack out of your arm then your chest is not high enough in your set up and carrying through out your lift, it's your form
If your knee and side hurt is because the weight is to heavy for your
Y'all got to understand Deadlifts, Overhead Presses and Squats is a Power lifting moves, that's for strength and if you don't train for strength and know what muscles come into play your secondary and third muscles and train them to become strong then abandon the lifts or stick to light weights, that's how people get hurt.
The beauty of Power Lifting is they will expose your weak parts of your body and you going to have to put in extra time to build and strengthen those parts
Mixed grip are used by the majority of Power Lifters because you can pull more weight then the overhand grips and you can control the bar with out it slipping like it does with overhand.
If people are pulling their bicep muscles it's because they got weak ass biceps and tendons and they need to train them more to become stronger to handle the Weight
Dan Green cane back from a Quad injury and tore his biceps deadlifting 900lbs, he used straps and the overhand grip, he pulled his biceps because his biceps wasn't used to the weight he didn't train no Deadlifts when he was injured. It's not the grips it's the muscles it had to get used to that weight again.
Also food, if you ain't eating right with lots of high protein foods to build your muscles will not get strong.
95% of the powerlifters at my Powerlifting gym used the mixed grip, I used the mixed grip because it feels better, I have strong biceps, I curled 175 before and have 19 inch arms, but I slightly tore my upper left pec during the pull, not because the grip my body wasn't used to all that weight so I started doing more chest workout
And also when you do the mixed grip your hands have to be perfectly placed, if not you going to sway, that's with benching, Overhead Presses ect, it's not because of the grips