The cage slips off because the cutoff lever is preventing it from pushing the trigger trolly forward. It's the little L shaped piece of metal behind the sector gear.
^do you see that little metal piece? every time you push the trigger trolly forward, that cutoff lever is stopping you from pushing the trolly forward.
In order to reset your cutoff lever you need to reassemble the gearbox fully and situate your gears so that the lever is facing down, and then shoot full-auto to reset the cutoff lever back to the default positions for semi and safe. Right now your cutoff lever is probably half way between safe and semi.
I don't know why this doesn't happen on v2's but only v3's. I only fix this shit i don't design them.
Rewatch the video and focus on that piece and you will know what i'm referring to.
Edit: remember to push the selector plate forward to enable full-auto or at least semi fire manually if you are testing this out of the gearbox. That might help lol.