The issue was not with rsync as I originally was thinking, but rsync passing through an error message. Running:

touch /mnt/1TBHDD/backup/.2022-05-22T13:09:38_db_backup.sql.gz.JVyRsP

returned:

touch: cannot touch '/mnt/1TBHDD/captionsearch.io/.2022-05-22T13:09:38_db_backup.sql.gz.JVyRsP': Protocol error

The error is because colons are not allowed in windows/CIFS filenames. Changing my naming convention away from those characters resolved the issue.

The issue was not with rsync as I originally was thinking, but rsync passing through an error message. Running:

touch /mnt/1TBHDD/backup/.2022-05-22T13:09:38_db_backup.sql.gz.JVyRsP

returned:

touch: cannot touch '/mnt/1TBHDD/myfile/.2022-05-22T13:09:38_db_backup.sql.gz.JVyRsP': Protocol error

The error is because colons are not allowed in windows/CIFS filenames. Changing my naming convention away from those characters resolved the issue.