Same problem with my Mi8 in version weekly 20.8.6/7, 20.7.2 and 2.7.30 !
bug reported here: https://xiaomi.eu/community/threads/20-7-30.56823/page-6#post-555107
but never view solution for this big problem of ROM.
i have installed a stable version MIUI 12.0 after format , work with 4G data...