I think your timeline isn't right. It is a fact that the SOHC came first, the DOHC came later.
There's no question there may be differences between the two engines, but your statements seem contradictory.
Who are these "some?" And why would the later DOHC variant retain the older cast iron block, while newer production of the older SOHC block were switched to an alu-alloy block?
That would make later production models of the F6A - which has always had an aluminium-alloy head - an all-aluminium-alloy engine, which is not true.
I have neither seen nor heard of an alloy block F6A. The all-aluminium-alloy engine you're thinking of is probably the K6A.
Why are they "very" prone?
It's true that aluminium has a much greater coefficient of thermal expansion than iron, but I'd expect a properly designed head gasket to account for the relative movement of the head compared to the block in those cases.
I'd imagine they might be more prone, objectively speaking, to head gasket failure than block+head combos made from the same material, but also that design of the head and the way it loads the head gasket has more to do with it.
My own engine is a 2-valve-per-cylinder, SOHC, aluminium-alloy head, cast-iron block F6A.
There are many variants. SOHC 6-valve, SOHC 12-valve, DOHC 12-valve, carburetted, EFI, non-turbo, turbo - but they are all cast iron blocks and aluminium-alloy heads to my knowledge.