When trying to take modulus to a negative number it fails. I've only tested with 4 and -1.

Steps to reproduce:
1. Create scoreboard "name"
2. Add target player "x" with negative score eg. -1 and selector player "y" with a positive score eg. 4
3. /scoreboard players operation "x" "name" %= "y" "name"

Result:
Command succeeds, but x is not changed. In my case where target is -1 and selectors is 4, target should become 3.