If the rank of the activity is 1 and your current rank is -8, the difference between ranks is 8 (remember 0 does not exist). Thus, progress earned is 10 * 8 * 8, which is 640, meaning you progress 6 ranks and become a -2 with 40 progress towards -1.

rank: -8, increase: 1, progress: 0, user rank was expected to be -2, confused here.

here is the formula in my understanding:
1-(-8) = 9
9*100 + 0= 900
-8 + 9 = 1
the rank should be 1 , any suggestion?