I had the same on my bike but it only happened when I upshifted clutchless under high acceleration with a throttle blip between shifts. When I then down shifted with the clutch, I got gears grinding or false neutrals. I won't be doing that again.
Exactly, me thinks there lies the problem. Unless of-course by blipping he means just closing and quicly opening in order to upshift. Opposed to actually trying to add a blip to the process.
In my experience missed gear changes are much less likely while doing clutchless as it's way more positive, if done correctly.