I've got the MI 5 3/64, but no problem with the Data Usage panel. I use two Simcards and have a datalimit on both enabled. They count perfectly for me, on 6.12.8. Will try .15 as well
Tried to install it on Mi5, but after install on latest TWRP, it seems to be in a bootloop. continuously this progress bar followed by the mi loading screen and then reboot and again the same. Waited for over half an hour, but still the same!