I can't think of a single open world game (or game in general, for that matter) that isn't significantly variant in texture resolution across assets. And in all cases the lower quality textures are more obvious on certain assets over others based on size and prominence.
Nevertheless, it's a two year old game build with specific performance standards in mind, particularly for consoles and optimisation for CDPR's first open world engine. Memory ceilings were obviously considered even on open platforms like PC. And with that in mind I still completely disagree with the implications in the OP: The Witcher 3 on average has excellent texture work, both in technical and artistic quality. In is extremely thorough and robust in quantity of unique textures and assets in a single scene, particularly foliage, which goes a long way towards textural density and variety at any one moment.
Obviously the overall quality could be better. It could always be better. As has always been for every game, especially popular RPGs on PC, where fans will step in and slap on their own technically higher quality assets via mods. Nevertheless, nuanced texture details in the vanilla build are still very impressive, especially on armours, fabrics, and characters. Pavement and tile work also looks consistently impressive.
I don't know why MGSV is being used an example, for that matter. MGSV's texture quality always looked significantly lower on a technically level across the board to me. Gorgeous looking game, but very conservative in asset/texture variety and resolution.
