Yeah this definitely sounds like a "Power mode" is enabled. It drastically changes the way the mouse buttons behave, exactly as the @Master911 described https://www.gpsoft.com.au/help/opus11/index.html#!Documents/Prefs/Power_Mode_Buttons.htm
I'm not sure what would be the easiest way to exit the "Power mode" in the Opus 11 (I'd make screenshots but I'm on Opus 12 so it's different).
See also: I hate power mode, but it loves me