This isn’t unusual in a blowback 9mm AR but surprising from a radial delayed setup, that’s the point, not to fire unless it’s locked.
You did right to send it back, a potentially dangerous situation.
Personally, I’d ditch the Armscor, I’ve not been impressed with any Armscor ammo. For me, brass Blazer 115 gr. Is just as cheap and more consistent, and for a few pennies more there’s much better options.
I think if I spent the $$$ for a CMMG I’d buy better ammo like MEN 124 gr. NATO. Which is far from the best, but cheap enough for cheap plinking.
I’d also avoid any aluminum cased ammo, IMHO just too soft for unsupported chambers. I have several blowback carbines that choke on Blazer aluminum but run fine on any brass or steel case.