Can anybody help me out with symptoms of short stroking in a semi-auto shotgun?

I've owned numerous semi-auto handguns and rifles, but never a semi-auto shotgun.

The gun's a brand new gas operated SDS AR T-02.

The first time I shot it (with full power buckshot loads of various types) I would generally get a stoppage on the second to last or last round.

When this happens, the shell stop seems to be disabled, allowing a round to exit the magazine tube, tying up the gun by lying on top of the shell lifter.

Playing around with snap caps, I noticed a similar behavior unless the bolt handle was FORCEFULLY pulled to the rear.

I'm thinking that it's possible that the gun is sometimes short stroking, preventing the action from fully cycling. As I said, the gun is new and that model has a reputation for requiring break-in. I've added a +2 magazine tube extension, the spring for which seems excessively long/powerful. While cleaning and lubing the gun, I reinstalled the OEM spring, which seems to work much better in combination with the extension.

As I've said, I'm new to semi-auto shotguns, having only POSSIBLY shot one once before.

Any advice would be appreciated.