I don't think you are using this custom ROM (so it does not really make sense as a bug here), but an hypothesis would be that your battery requires a replacement.
@kenhill If you did not update your ROM recently, I think it is linked to this announcement
https://xiaomi.eu/community/threads/migration-of-ota-metadata-source.72743/
@sirrixofficial Again, you are posting in the install guide of a custom ROM, this is not the best place to ask detailed questions about updating your official ROM. Gogocar62 gave you the link to the official website, they have a forum there.