For anyone wondering, workaround for us was:
1. Downgrade read only slave server to mysql 8.0.19 (basically drop 8.0.20 and download packages from https://repo.mysql.com/apt/ubuntu/pool/mysql-8.0/m/mysql-community/);
2. Restore the latest backup;
3. Connect this one as read only replica again
make backups out of this replica server