Any remaining progress earned while in the previous rank will be applied towards the next rank's progress (we don't throw any progress away).
The exception is if there is no other rank left to progress towards (Once you reach rank 8 there is no more progression).

I have a same problem. My code works in all conditions. I can't uderstand why expected progress have to be 21, what the ** 1 has added?
Initial rank is -8, progress is 0;
After applying rank of -1:
delta = -1 - (-8) = 7;
progress += 7*7 * 10 = 490;

userRank = -4, progress = 90;

uniqueInOrder('aabcdeb')

function return 'b' 2 times