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The behavior your are experiencing is, indeed, the expected one per our documentation (see "Backup archive name" on that page). As I have written in there:

Please note that -[RANDOM] will be appended automatically to the Backup archive name if you are using the default backup output directory and you are not already using the [RANDOM] variable in your Backup archive name. This will happen automatically at backup time. You cannot override this behavior because it is a security feature.

I will not give you an option to override a security feature for the same reason you can't remove the beeping sound when you don't fasten the seatbelt in your car. It's a security feature. If you disable it bad things can happen. They will not necessarily happen but they can happen. Much like you're wearing a seatbelt in case you get into an accident, not because you know for sure that you'll get into an accident. Seat belts save lives. Random characters at the end of a backup archive's name save sites.

Of course you can have these characters not added by moving your backup output directory to a non-default location as it has been documented for the last 13 years. Moreover, for the past 12 years, every time you go to Akeeba Backup's Control Panel with that backup profile active or take a backup through your site's backend with that backup profile active a warning about it appears with a link to a page with the same information and with a bit more detail. Please follow that link and do as it says. As long as that directory is not accessible over the web (Akeeba Backup will check it when you save the Configuration page) the -[RANDOM] bit will not be appended to your backup archive's name. Your problem is solved AND you have increased the security of your site. Killing two birds with one stone :)