I've actually been able to duplicate this with my 500A and 590A1 when loading a shell directly into the chamber and not from the magazine. Sometimes its just the way the shell falls into place combined with the action being pushed forward.
Not gonna swear this is it, but maybe try loading like this then shuk the shell back out, and repeat. do it slowly and deliberately and see if it happens again, if so, try to make note of the process that causes the jam.