During that time, your data is encrypted but you don’t know because when you open a file, your computer decrypts it and shows you what you expect to see.
First time i hear of that. You sure? Would be really risky since you basically need to hijack the complete Filesystem communication to do that. Also for that to work you would need the private and public key of the encryption on the system on run time. Really risky and unlikely that this is the case imho.
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.