Let's try doing a clean install and delete all the cache of the program. Uninstall your software first, then go to the folder location below on your computer then delete the Mixpad folders:
C:\ProgramData\NCH Software
C:\Users\(Your computer profile)\AppData\Roaming\NCH Software\Mixpad
If you can't find the AppData folder, set Windows to show hidden files and folders with the steps from this link: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files.
After deleting the Mixpad folder, restart your computer then install it again from the next link: https://www.nch.com.au/support/supportcontact.html?software=MixPad&support
That should do it. Please try it again after that and see if that fixes the issue. If you still getting problem with the Option of Vertical Zoom, please send the feedback to the Technical support Team: https://www.nch.com.au/support/supportcontact.html?software=MixPad&support