I've noticed this issue in Eevee (and Look Dev) but NOT in cycles, where I've UV unwrapped a section of my model, used exactly the same shader nodes where the two materials would meet, but there's a strange shadowing effect outside the edges of the UV unwrapped area.
Is there a setting to fix this that I am as yet unaware of? See screenshots below. First one is the Eevee viewport render (you can see the edges of the unwrapped area), the second is the Cycles viewport render - with no issues.
Thanks for any help!