Reversing lights on when not in reverse gear

[Gaz mr-s]

My pfl needed a new gearbox. A 6 speed SMT box converted to manual operation has been fitted. It's been back on the road for about 3 months.  I had never been followed by anyone I knew until a few days ago. The reversing lights are coming on when the lever is pulled out of a gear, & again on the way into the next gear. A fraction of a second each time.
They do work properly when in reverse.

Must be a simple answer, ....hopefully a simple fix....