Usually when a fill probe wants to eject, it’s because one of the O-rings is sealing and the other one is not. Wipe them clean and check them with a loupe for any debris or a bite taken out of the perimeter. Check the inside of the port in the gun as well for any debris or scratches or gouges that could be interfering with the ability of the O-rings to seal.
If all looks okay, lubricate the O-rings with silicone grease or silicone oil and try again.
If it continues to eject, I’d be inclined to try new O-rings in case the originals have an imperceptible flaw.
Another thing to check is that the probe isn’t seating too deeply (or too shallow) such that the outboard O-ring (or inboard one) isn’t staying completely captured inside the port as you begin pumping.