Raytracing in Unreal is not what you would call a real path tracer(Arnold, Octane, etc) but it’s a hybrid of things, there’s the Path Tracer mode but it’s kind of dumb at the moment. Translucency(which involves Refraction) is coded separately from the rest and comes last in deferred rendering pipeline(even GI comes before iirc), basically watching through a translucent object it does what a path tracer would do and rerenders the whole scene(therefore no denoised shadows or GI). The raytraced shadows where denoising happens are totally disjoint with translucency pass.
GI is also weirdly implemented as a lot of shading models(e.g: Eye, Hair) have no “diffuse component” in their code and the GI implementation uses that and therefore GI basically don’t affect them at all.
Screen space refraction is actually very good alternative and you have denoised shadows/GI, of course it has limitations 