Thanks for the reply, that option is not checked. This issue only appears whilst I am using MSFS2020, it does not happen at any other time.
In MSFS 2020, as an example, if I scroll up or down on the secondary pc, the cockpit will zoom in or out, or if I am on the world map looking for a location the world will zoom in or out.
It is not restricted to zoom, if I click the left mouse button on the secondary pc, this is also felt on the primary pc.
It is acting as if the checkbox you highlight above is checked but again only whilst using MSFS2020
Hope that clarifies a little more.