Experience from monthly Disappearance

Last night, I was playing a few games to finish the monthly challenge, and I finally did manage to do it. I got myself about 2/5 through the level I had gotten through, SR-58. The next day, I was looking at my stats from the site, and it said I was still in the same level as I was before I leveled up from SR-57 and thought that it was still behind or something. When I got home this afternoon, sure enough, the experience earned from the challenge was lost. What happened?

I know I did’nt hit the cap cause I only play a few hours a day.