Ply is notorious for bending up or down on unsupported joints. It can be significant enough to stress any felt or membrane over the top of it. I'm surprised it is not a requirement up there. Especially as that looks like the trusses are at 600mm spacing.
For sure, the more support for sarking sheets the better really, although sarking is quite rare in England - although a great idea in my opinion.
My holiday cottage in Seahouses is though, using tongue and groove floor type boards.
As for new homes build quality.......you could spend your like discussing this one!
Nothing to worry about in your wee hoose anyhoo.
Have a good evening!
John
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