Yup, you got clutch drag. Nice shifting between gears is a natural but trying to put it in neutral when the secondary shaft is stopped and the engine is spinning at 900 rpm absolutely needs freedom between the plates in the clutch.
First off, you have checked the clutch play and still no joy. If that's the case the most logical thing to consider is the oil you're using. If it's not MA spec oil, there's your problem. some oils have friction modifiers in them which will cause the clutch to slip, others have wear inhibitors which will help out with minimizing wear but won't have an adverse effect on the clutch. If you're oil is MA rated AND it's fresh, go to the next check. If it's old oil and I don't care if it's only seen a couple of hundred miles, because even old that's just been sitting in the crankcase will age and as it does will thicken up, you need to change it.
Second, when you checked the clutch play, did you look at the clutch lever under the cover on the left side? Is it 90 degrees to the cable? If not adjust that first, then adjust the cable. next, operate the clutch lever and see how far it has to travel before actuating the clutch lever (under the cover) I've seen old clutch cables and cables that are not routed correctly get adjusted by feel and when operated, half of the travel is used up in taking up the real slack. If that's the case, look at the ends of the clutch cable outer at the ferrules on the ends. If they're not solid, then replace the cable.
Third if that's all good then the last area to look at is the clutch hub and clutch housing. quite often the plates will wear little notches in the hub and the housing and the clutch plates can get hung up in these notches holding the friction plates and steel plates loosely together and that will cause drag. The notches can't be dressed out BUT if there's burrs beside the notches, you can dress those off with a fine file and that will help, not cure but help.
One last item that comes to mind. Some guys with little hands prefer offset clutch levers or other guys just like the looks. In any event, if the clutch lever is not the stock lever, you may not have enough travel period.
That's it, fingers are sore. Good luck.