Water mask as metallic-roughness texture


I’m converting our self-made water mask textures to metallic-roughness textures in the 3D Tiles dataset we created ourselves. It seems to work well in Cesium for Unity, not at all in Cesium for Unreal and in Cesium ION it doesn’t reflect the sky as in Unity but it reflects the sun.

However, in ION which is the focus of this question, the edges between fully metallic and fully rough pixels are lit up very prominently, apparently due to mipmap filtering:

So I changed to a NEAREST minification filter and the problem disappears, but is replaced by an (expected) flickering from a distance instead. Especially in Unity where the sky is reflected. See the jagged pixels in the reflections in ION here:

Any suggestion how to approach this problem? The ideal solution would be if ION or CesiumJS behaved like Cesium for Unity, reflecting the sky without any filtering problems if minifcation is set to LINEAR_MIPMAP_LINEAR. Or if anyone has a better idea for how to convert our water masks into realistic water?