I have noticed that some plastic filler rings snap in. I think it is because the fit is too tight, causing the rim of the cup to snap when it gives way. The mouthpiece on mine is a perfect fit on the cup. It goes on easy and comes off easy. I have only tried it on two though, so it may not fit that well on all E2's. So far, mine do not leak.
Heck, try one of yours and see. It won't take but a few seconds to remove the ring and cram the mouthpiece down in there.
I have only lost one of mine during a burn. I don't think I would have lost it had I cleaned it sooner. The crud on mine was very hard. So hard that the coil could not burn it off. Naturally I kept trying and it died a tortured death. Look at the photo of it a few posts back ----> #
34. That's what it looked like after all those burns. I am cleaning mine more often now and getting a clean burn. We are still learning about these things, so there is no telling what we can expect from them.
Edit - I forgot. On one of mine, I shortened the tube and now the end of the mouthpiece is exposed.