That account was set up 7 years ago and on that phone (S7) 4 years ago. In a different country. With different phone number.
It was working just fine until now, and only just now I realized that the phone numbers didn't match.
After changing phone numbers on the old phone WhatsApp, the new phone WhatsApp found the backup and successfully restored it.
From MI Mover review
APPS THAT CAN BE TRANSFERRED USING MI MOVER
The Mi Mover can transfer system apps and third party apps these apps are mostly from Xiaomi and games that doesn’t need personal data.
APPS THAT CAN NOT BE TRANSFERRED USING MI MOVER
However, Mi Mover cannot transfer apps that have personal data. For example Facebook, Shopee, and Lazada. ...