https://bugs.winehq.org/show_bug.cgi?id=56026
--- Comment #1 from alexandros.frantzis@collabora.com --- Thanks. Please see https://gitlab.winehq.org/wine/wine/-/merge_requests/4695. This adds support for win32 xbutton1 and xbutton2 (i.e., back and forward). Let me know if it helps.
What's the functionality of buttons mouse3-mouse6 (do you start counting from 0, with mouse0-mouse2 as left/middle/right?) on your system, e.g., with WineX11?
Also, if you can provide it, it would be helpful to get the output of evtest for each of the buttons on your mouse, to see what kind of evdev codes you actually get for each button.