
IMHO opinion the issue is not a weakness thing but rather a technique/load issue.
You are basically front squatting with a back squat technique… you lean forward at the trunk, obviously it is impossible to maintain a tight upper back if you do that.
Look at my torso angle in the picture… it is almost perpendicular to the floor. THAT is a front squat position. You are basically doing a powerlifting squat with a front rack!
My guess is that either…
(1) you are too tight in the hip flexors to maintain that upright posture when squatting down
(2) have inefficient levers for the lift (unlikely)
(3) simply have not practiced the mechanics of the lifts and just do a normal squatting motion with a front rack
(4) are using too much weight for your quads strength, so the body tries to compensate by bending at the waist to involve the lower back more in a dynamic fashion
My recommendation is to learn the front squat movement pattern. Do not use it as a training exercise until you can do it with an almost perpendicular torso, and certainly do not use an advanced method like chains.
Oh yeah, from the video I cannot see if you are using a clean rack or a crossed arms rack. The later encourage rounding the upper back. Only do front squats with a clean rack (or with straps).
A good drill to learn the proper torso position in the front squat is the Sots press. Go into a full front squat, and press the barbell overhead (like a military press) while staying in the full squat position. It is impossible to do if your torso is not upright/perpendicular to the floor. Start with only the bar, it should be challenging enough!
BTW a true Sots press is done with the barbell on the front of the shoulders (clean rack) not with the bar in a back squat position.
Oh yeah, Viktor Sots (who basically invented this lift) did 160kg x 3 reps and 180kg x 1 rep on that exercise. He was also the first “big guy” to use the squat/power jerk style in international competition, doing 232.5kg in competition (and reportedly 260 from racks in training).