OP, Sellier and Bellot has had reputation in the past for causing such malfunctions...especially in 45 ACP. Before condenming anything, including yourself since you’ve had no issue before doing the drill on other previous occasions, try a couple boxes of your stashed you KNOW works in your pistol. The only thing that has changed is the ammo.
The no mag extractor test is a good one. But if the pistol is short stroking from too low velocity or too slow a powder (weak recoil impulse) it will fail be ause the momentum on the ejector is weak.
Personally I’ll use S&B ammo if the price and availability is good - but strickly for blasting and practice. I use other proven brands and loads as my go to ammo.
I SHOULD have used my own but didn’t feel like driving all the way to Marriottsville today, so I popped over to Gilbert this morning and bought a box of Fiocchi (you take what they can give you there). Did the drill about ten times with no issues. I did focus more on properly slingshotting the slide in case I was slacking there doing the drill, and focusing on my grip. I guess I could have tried a deliberate weak grip shot to see what happened.
At any rate I’ve never had an issue with a a magazine loaded and real-world conditions are what counts.
However I have definitely proven to myself that I should go easy on the coffee on mornings when I plan to shoot, because I was punching through the same hole on Wednesday and I suucked today.