Here's an odd one. Anyone seen this before?

I was shooting an XDS .45 for the first time. I don't normally keep .45ACP around so I bought some cheap WWB at the range.

Got through half a box with no issues, then this round made a little "pop" then a "hiss". A little plume of white smoke wafted from the ejection port.

I figured it was a squib, expecting to find the bullet halfway down the barrel. Waited a minute, checked the chamber and found this.

It was covered in black soot and had clearly shot flame into the side of the chamber.

Very strange indeed. First time I've seen one like this.