I have a very stiff left SI joint and pretty much my whole lumbar spine is stiff. The icing on the cake was/is antrerior pelvic tilt from shortened hip flexors.
Not all powerlifters use extreme back arch on bench. Some people find they just can’t arch that much on the bench due to back problems or flexibility issues. If you’re finding it hurts your back that much you’d probably be best to change your style.