Today I have just had to go back through all our old posts and change references of Istvaan 3 to Isstvan 3. This puzzled me as my brother and I could have sworn it was Istvaan. I looked into it some more by visiting some Wiki sites and other forums who have also had the discussion.
It does appear that originally it was spelt Isstvan. At some point a GW staffer must have slipped up and changed its spelling. So when Black Library got involved it got changed back to its original spelling.
So as it is referenced as Isstvan in Black Library and Forge World I deem it the correct spelling going forward.
I recognise my failing and will be sure to correct it.