Thanks you saved me some time researching the warranty issue.The mouthpiece is classed as a consumable part and is not normally covered by warranty
I am going to purchase a replacement. I will keep the one I have as a spare. It works well using tape to hold it in place. In my case I believe it had something to do with heat softening the glue and the strength of the magnet. Removing and replacing the cap when it is hot may have added enough force to free the glue bond. I was high at the time and don't recall what I was doing. It is possible there was a poor glue joint but looking at it the glue seems to have been evenly spread.The magnet is very strong and can cause the mouthpiece to snap fairly hard onto to the body; I bought a new mouthpiece and now I'm very gentle with it when placing onto the main body. No problems so far.
I wouldn't use any glue or epoxy because of fear of inhaling the fumes.
I have changed my vaping style. It is easy to sense the amount of heat in the mouth piece and just take a few less tokes as I feel it getting hot. And I will let it cool down a bit before removing the cap, another advantage of owning two!
I am going to pass on glue. J-B Weld has an epoxy for high heat rated at 550°F/287°C however it may not work well with plastic. And yes I would worry about unwanted fumes.