Pause and watch it frame by frame or best you can. The empty is partially ejected and he actually shoved it back in with the fresh mag.
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Could be. Glocks do have mushy mag release buttons and empty poly mags are often slower to drop than steel mags unless you sling / flip them out. (I've never seen a cop do that. Maybe some get trained to but I'd bet few)
Or he just missed the button and went to step 2. Shit happens I'm not picking on him.
I'll bet if he had one regret it would be that he didnt stay ON the guy when he saw the shotgun OR that once he decided to fall back he wished he'd gone low and towards the passenger rear of BG's truck and maybe kept going for better cover if available.
Falling back in the direction he did was his biggest mistake.
But, shit happens and he kept himself alive, in the fight, and shot up the BG. That's the good thing about these videos. To learn what went well, what didnt, and how to improve training.
I have seen people in training classes grip the gun so hard that mags don't drop free. In the stress of a shooting situation I have no doubt he was squeezing that poor gun hard enough that there are frames back at the factory feeling it.