Thank you for your reply.
No, it's not just in the viewport, mostly it happens on some of the same objects.
They usually always have multi-sub mats.
It's also hard to recreate this problem and I don't know when/why it appears.
I have the model and some testrenders where the error is visible, I've reapplied the material quite some times (about 10-12x over the project duration).
Was just wondering if this was a more common issue...
The project has finished by now, so it probably won't happen again as I've never had this problem before