@ EternalLie
Unless you have MS XP/Vista there should be no reason why you can't update your program version to the latest version. So why it ends up showing 18.8 as the program version is petty strange. More so given the SHA256 is identical to the one you gave for the off-line installation file and the one I downloaded for 20.8.2429.
Since you have installed the off-line installation file, did you do a system restart ?
That would be to complete the installation, especially if you had a previously installed version of avast ?