Reaper Preferences/Editing/Mouse
Uncheck the box for "Control + left click emulates right click"
This is "hidden" in that it's possibly the most confusing language possible!
Checked is normal mouse operation with control-click (right-click) giving you contextual menus.
Unchecked disables the control key from the OS and lets Reaper use it for an additional modifier key. Understand that you will give up contextual menus which are used often in Reaper. Pick your battles.
There's no emulating going on whatsoever. (Right-click has been a shortcut for control-click since forever.) This is probably messed up from the key swap between OSX & Windows (Command key vs. Windows key)