It seems that translucent shaders set to forward shading are totally broken in UE5.4.
I had a scene in UE5.3 which had subsurface on translucent objects for curtains that were forward shading, and they work fine.
I migrated this over to UE5.4 and they are just totally white and completely blowing out the bloom, so yes very broken, I even removed everything from the shader and it was still broken when set to forward shading, set it back to deferred and it was fine, unfortunately many shaders require forward shading on translucency, the material editor also gives me an error if you try to use the noise node, and breaks the shader, this also worked in UE5.3.
I am sticking with UE5.3 for now as it seems way more stable, disappointing as I held off getting 5.4 until the official release as I know there can be bugs in the preview versions.