Check out the Wikipedia article :

https://en.wikipedia.org/wiki/Fasciation

It's pretty comprehensive and should provide you with the info you want.

Your plant might be predisposed to fasciation. Whether your growing conditions are somehow conducive to stimulating your plant to display this characteristic would be interesting. Btw, don't believe this would technically be considered "stem" fasciation as what you are seeing is a petiole not the stem (the stem being the AV's "trunk" while the long portion which joins the leaf to the stem is the petiole)

Fasciation is definitely "fascination" in my book! Very interesting. Rosie

I read somewhere that often a plant sports if it starts throwing such leaves. And I bought a young plant 'Ian-Fuete' on ebay - that always tried to grow these leaves - and it was a weak plant that eventually died. So - it is not a good sign.

I wonder if babies from such leaves would grow likewise..? Because that might be interesting to have.