Reproduction:

1) Create a PBR material

2) Set Blend Mode to Mask

3) Set Alpha to 1.0

4) Set Alpha Cut off to 1.0

This produces a noisy/flickery surface. This is likely due to floating point rounding errors in the shader.

glTF spec says about the material.alphaCutoff - "If the alpha value is greater than or equal to this value then it is rendered as fully opaque, otherwise, it is rendered as fully transparent"

So with both values being 1.0, it should be fully opaque.