i have sevral fig trees growing in the same area - about 15 or 20 feet apart. the leaves on one look strange but only on the north side of the tree.  they re growing in composted horse manure - 1-2 years old - the tree next to it have no issues ... i put in lime, K, & P   

That's FMV. The trees may grow out of it given time.

FMV.

Could heat stress also cause this?

One of mine has leaves exactly that thin and twisted. I put it in ground and all the new growth is more normal. In the past others have shown slight symptoms, leaf mottling and such,  and outgrown them but didn't look so severe. It's much more of a worry with a small tree.