Was the empty case which became caught in your ejection port (base up?) a Winchester USA brand ("white Box") case? Furthermore, was it a 115gr loading?
Okay, no way to know what happened without having been there to have inspected the gun before it was resolved & cleared.
I could, however, imagine a combination of events that could easily result in something that could match what I believe I understand you saw, based upon your description.
New-ish shooter, new plastic pistol ... and a subcompact plastic pistol, to boot, maybe?
Did you have a LOCKED wrist while shooting each and every shot? A reasonably firm grip (think healthy handshake, not "deathgrip")? Both a firm grip AND a locked wrist - in 2 planes - are really required in order to experience optimal functioning with a centerfire semiauto pistol ... and especially while shooting loads that may be loaded on the lighter end of the normal power level.
IF ... and I mean IF ... a momentary combination of an unlocked wrist AND/OR a lighter recoiling round occurred, it might reduce slide velocity/run just enough so that the fired round's empty case was caught by the ejection port of the slide. The "upside down" case position wouldn't be all that unusual, depending on the sequence of events.
Now, the empty case being in the ejection port could prevent the next live round from being fed into the chamber, but
it might be almost out from under the magazine feed lips. It wouldn't take much to have it become displaced out from under the lips (such as manipulating the slide).
Manipulating the slide while turning/tipping the pistol could easily result in the empty case and the live round to fall free. The "next top round" could also have been moved just enough forward when the round which failed to to feed was dislodged (at some point during the manipulation of the pistol & magazine).
Stripping the magazine from the pistol might have easily resulted in the empty case and dislodged (but not fed) live round falling free of the pistol (and the partially dislodged/displaced
next round remaining in the magazine, but slightly forward).
Now, has THAT magazine been dropped by you onto any hard surface since you've unpacked, inspected and cleaned it? (Yes, brand new magazines ought to be cleaned and inspected, and then reassembled DRY, having wiped out any packaging oil.)
Do the lips appear normal and undamaged (from production)?
Might have been your grip (unlocked wrist) ... combined with a light load (Win USA?) ... and just the way things worked out when the gun was being fired.
Bear in mind that brand new guns often have tight/stiff recoil spring assemblies (RSA's), and this can also contribute to a potential reduced slide run ... when a light load and/or an unlocked shooter's wrist occurs.
If the gun appears normal ... and this had happened with one of our shooters (at my former agency, where I remain involved as a firearms instructor), I'd caution the shooter to maintain a normal firm grip with a locked wrist ... and try more rounds through the new gun.
Things like this can happen with new/newer shooters, though ... and adding reduced size pistols with reduced weight slides and faster cycling speeds ... and possibly lower recoiling ammo ... can often lend itself to more potential of something like this happening.
I can't know what actually happened in your case, since I wasn't there at the time, but I've seen things like this happen often enough that I'd not be surprised to see it happen, if those circumstances and conditions I mentioned were present at the time.
Just some thoughts.
