When I attempt to set opacity on a mesh with multiple materials, the other materials are overlapped instead a clean opacity. I am using this for rendering a building in “ghost” mode during placement.
I did try adding TemporalAA and it looks ok, but not clean. There is an odd flickering with the mesh.
I set random to 0 and it works fine.
Transparency in unreal is a fake thing without priority sorting.
Essentially, split out your mesh so that the transparent material is its own thing.
Look into the old car sample stuff where the windows are done with one mesh that has 2 differently set transparency types to get it working right.