Um, yeah... false neutral on a Ducati... so what else is new?
I don't mean to be a smartass - false neutrals are pretty common on a Duc. I've had them on my '74 750 and my 2000 996 and everything in between. I usually see them between 5th and 6th but they can show up elsewhere. In my case it's usually because I'm getting lazy with my shifting.
It COULD be something like a bent shift fork, but it also may be you're not being positive enough on your shifts. You don't have to slam it into the next gear, but you should do more than just put a little pressure on the lever. Half assed attempts will make it shift most of the time, but not all the time...
In any case, it's not something I'd worry about unless you are being very deliberate with your shifts and still having issues. (On older bikes, as in bevel era, there's an adjustment you can make too, I'm not sure if the newer bikes have it? I don't remember ever reading about it and certainly haven't had to do it)