Being banned on Pokémon GO quite common if the company found you cheating by spoofing and even playing more than two accounts.
Apparently, many reports made over on Reddit that Niantic has banned a lot of Pokémon GO accounts and shared the same common; Xiaomi Phone users.
There are few speculations of the bans, the MIUI operating system is classified as unauthorized third-party software that violated TOS. This is because the OS itself allows users to play two accounts simultaneously. The bans started when Xiaomi rolled new software updates for few Redmi devices like 5 Plus, Note 5, Note 7 and more. Besides that, some claimed it was the AppBooster in the MIUI 10 that caused the ban.
Or it could be some technical problem but so far, Niantic has yet to make any announcements about the ban issue but they raised up an issue with some changes in Android Q for Ingress game that has similar gameplay with Pokémon GO.