Does the well seal compress around the poly pipe enough to hold it in place? Or would the shoulder of whatever adapter you use sit on top of the well seal.
I've never seen 1" or 1.25" PE pipe not sealed in a sanitary seal. And in 20+ yrs I've never seen anything but PE pipe in a seal. Many of them had been buried from 4' to 10' deep up to 35 yrs and more.
So a better question is how can it not seal? Well you could run 1" pipe in the 1.25" hole, THAT won't seal.
And if you think about it, hose clamps are not going to go down the hole in the sanitary seal are they? So the seal is against only the pipe.
I do not believe PE with an insert fitting in it is going to go down the hole without clamps on it even before you tighten the seal as you are supposed to; evenly (all 4 bolts in an X pattern) and none overtightened, just tightened to seal and a half turn or so more and you're done. No lubricant on the casing, pipe or seal.
Any type pipe will be sealed very tight by the sanitary seal as long as it is IPS (iron pipe size) pipe. That's because the holes in the seal are sized for pipe, not tubing. And since I'm thinking about it, possibly better than a nipple; especially compared to 160 or 200 psi rated PE.