On a recent trip to Fraser Island our 2009 Mazda BT50 manual (120,000 km) started jumping out of first gear on downhill grades. There was no throttle, just engine braking.
There's no problem when you apply a bit of throttle, it only jumps out of gear when you are idling down and doesn't happen if you have light hand pressure on the gearshift. Is this an issue for this box?
There are plenty of reports of the BT50 gearbox jumping out of fifth gear, but we haven’t seen one of it jumping out of first. It could be wear, although it’s very low kays for wear to be showing up. Take it to a gearbox specialist and have it checked.