Ours has had an ITT split since the day it came from the factory. There is an "allowable" limit - after all, you would never expect them to be completely identical - and our was close to, but inside, the limit.
And indeed, 1800 hours later, the split is the same as always, including no change after the HSI.
The indication of a fault is when either of the ITT's change, so you should really be doing some basic monitoring, but the underlying split is nothing to be worried about.