Try (always wipe dalvik/cache). If it doesn't work, format, reboot to TWRP, transfer rom-zip, flash. Make sure you have a backup if you want to save anything.
Running the same setup on Mi8 and have no issue. Have you disabled power saving for Mi Fit-app? If not, try that. And update to the latest rom of course if you're not running it.
Same here and works fine with smartwatch and my car (Mazda), location/on body works too but I'm not using those. Don't know why some say it's not there.