I don't believe the G3 chucks have ever had a sealed back. The other Nova chuck I have, SuperNova, does have a sealed back. I would guess sealed or not sealed would be at least a big part of the problem. However, I have to say that I have 2 G3 chucks, one I got with my first Nova midi lathe and the other in a special deal on a set and I've had zero problems with them. Maybe you just do more turning than I do. I don't believe that Nova's chucks are the best available, but they have been very reliable for me. The only time a blank has come off has been when I didn't get the face quite right or the tenon larger than I'd like and those were when I first started turning. I do clean out the back once in awhile on my G3 chucks but don't remember ever dissaembling them to clean.
My G3 are not sealed. The Record Power SC4 is, but the G3 are not.
I haven't had a blank come out of the chuck unless I had a bad catch. The main problem I have had, is the chuck functionality being impaired by captured compacted dust. Its happened repeatedly since I've owned them, and its a pain to deal with. I've had other bad experiences with Nova/Teknatool, and I'm just at a point where I need to be done with them. The annoyance factor is just too high. I like tools that don't get in the way and just work. Its been that way for my photography, my astrophotography, my computer work, and now...inevitably...my woodworking. I don't need "the best", but, I do prefer to work with "better" and anything that doesn't get in the way or hamper what I'm there to do.