Are you appending to an existing backup file? Try backing up to a new file.
Your backups on the existing file appear to have been created using a BLOCKSIZE of 65536 bytes.
BACKUP DATABASE model TO DISK = 'c:\test_01.bak' WITH INIT, FORMAT, BLOCKSIZE = 65536
BACKUP DATABASE model TO DISK = 'c:\test_01.bak'
The 2nd backup fails because the default BLOCKSIZE for a disk backup is 512 bytes, raising the same error you encountered.
SQL BAK Explorer - read SQL Server backup file details without SQL Server.
Supports backup files created with SQL Server 2005 up to SQL Server 2017.