I’d love to know the answer to this as well.
I’ve had the same issue on my first climb of both Rainier and Hvannadalshnúkur in Iceland. In my opinion it comes from a salt deficiency or electrolyte imbalance. I was drinking Propel mix in my waters but I think the salt capsules would be ideal. Keep in mind I didn’t have these issues until the 2.5-3hr mark. How much are you sweating on the approach? Also try to look into rest stepping. This will help prevent contracting the leg muscles too much. Each time I had the cramps, they were solely in my quads and hip flexors. I too felt fine after the first day, maybe because of the salty foods and soup you have your first night at camp. You definitely are not alone in this issue.