I'm unable to reproduce the issue. I've assigned the "Internet: Page backward" command to an extra button that I happen to have on my ASUS ROG GX1000 mouse and it works correctly here in Vivaldi 1.8.770.54 (Stable) / 1.9.804.3 (Snapshot) (both 32-bit on Windows 10 x64). I'm using the built-in "forward/backward" buttons all the time and they always worked correctly in Vivaldi.
Even in the (not-so-distant) past, when I still used the worst piece of hardware I've ever had (Razer Mamba 2012) along with the worst piece of software ever created by humankind (Razer Synapse), the built-in "forward/backward" buttons worked correctly in Vivaldi, always going 1 step forward/backward at a time.
Do you say that all the mice that you've tried go backward two steps in Vivaldi? That's strange... Perhaps there is some problem tied to your specific configuration of your hardware/software/OS... 🤔