Assuming you are not short shifting the lever with lazy foot...
The drum rotate pawl and/or the shift drum pawl engagement features could be worn.
The picture shows the gear selector resting on the flatted star tip. This is the neutral rest position. When in gear, the drum has rotated so the detent round bit rests in the valley between the star points. A false neutral would have the detent positioned at one of the non-flatted star tips and not rolling into a valley.
This mechanism is behind the clutch, btw and can be examined without pulling the engine.
Assuming the detent and shift pawl parts are operating as designed, the next inspection area is inside the cases (engine pull), where you can inspect for play at the shift fork pins resting in the shift drum grooves. And then, the shift fork width in relation the dog slider mating groove.
One thing easily checked is that the shifter arm itself is firmly tightened onto the shifter shaft splines. Slop here can aid in short shifting, wich doesn't allow the gear selector detent to find its proper valley.