I doubt that this is the case, whether it is encrypted or not. The complexity and risks involved with decrypting it on the fly is really unrealistic and unheard of by me (have not heard of everything but still)
Also the ransomware would also need to differentiate between the user and the backup program. When you do differentiated backups(like restic) with some monitoring you also would notice the huge size of the new data that gets pushed to your repo.
Edit: The important thing about your backup is, to protect it against overwrites and deletes and have different admin credentials that are not managed by the AD or ldap of the server that gets backed up.
You want your backup functional even if the system is compromised so yes another system is required for that, or through it to the cloud. Important that you do not allow deleting or editing of the backup even if the credentials used for backing up are compromised. Basically an append only storage.
Most Cloud Storage like S3 Amazon (or most other S3 compatible providers like backblaze) offer such a setting.