There’s no reason why partners on a collaborative project should get free access to each others’ sovereign technologies. That goes for F-35, GCAP or FCAS.
Partner countries can still get the benefit of these technologies, but these should be implemented by each country inside the overall GCAP design… ie. Italy shouldn’t expect some kind of open book and to free ride on a decade plus of UK R&D in stealth, engines etc.
There’s a parallel here with FCAS/NGF, where Dassault says it isn’t learning anything from its German & Spanish partners (as they don’t have much to bring to the table - at least in terms of platform design and stealth), but Airbus D&S is frustrated that they aren’t getting access to Dassault’s proprietary studies and prior R&D.
Of course no one would expect this level of full technology sharing from the US, even as an F-35 partner nation, but it seems like some expect BAE and Dassault to be different from Lockheed Martin.