unity – How do I remove the shinyness from the terrain when using an orthographic camera?

I am having an issue with terrain having a sheen around edges when I set the game camera to orthographic. Changing it to perspective, the shinyness goes away:

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I have tested with textures from the Unity terrain tools sample pack, as well as purchased textures, and they all have the same problem. The textures have diffuse, normal, and mask maps assigned. Metallic and smoothness sliders for the textures are both set to zero.

I have read up on channel packing for the mask map, but even setting the metallic and smoothness channels to black makes no difference, it seems entirely dependent on camera mode.

It’s important that I keep the orthographic view for my game, is there any way to fix this?