Have you done something to isolate the config files between the two version (like creating a config directory under 2.80 in the blender installation folder) that would explain how the two versions might be looking at different settings?
I run exactly the same setup and never saw an issue like this, so I think you have something messed up in the config or your environment somehow.
Try File->Load Factory Settings in the old version and see if that makes a difference. It could be something in the keymap got messed up for example. This is easy to do if you play with the Assign Shortcut right click menu option, where if you click on the highlighted blue press-a-key button it assigns left-click to the function.