Ok. So it sounds like a cable issue, rather than a switch issue. There are gear position sensors at the bottom of the gear selector, in addition to the gear selector switches on the gearbox.
I suspect the cable has stretched slightly, causing a mismatch between the gear position sensors below the gear selector, and the switches at the gearbox end.
I'd try moving the cable a fraction at adjuster, and try it again.
I'd also check for corrosion on the multi-plug of the lever position sensor below the gear selector, as this has happened to mine in the past.