It happened to me when I grew a GSC cross, so it's in those genetics. I had seen a pic of a leaf flower posted on an online forum a few years before GSC was created, so it could be in other lines, too.
I’ve read somewhere that it’s Chinese landrace genetics in the mix. I had one a few years ago. Same plant threw a 19 fingered leaf too. I can’t remember what strain it was.
This "oddity" is also starting to appear on other leaves on the plant. It starts as a flat, lighter green leaflet-looking thing in this photo (center right).