Passing Specular BSDF through Shader->RGB when SSR is enabled causes shading other than ambient world lighting to be removed, not just the SSR reflections. The same happens with Glossy, Principled etc.
After bisecting, this was found to be caused by https://developer.blender.org/rB7f7e6830991 but it's a large commit so I didn't dig in and find exactly what was responsible.
(If I've already found the commit/cause should it still be opened as "Needs Triage"? Would marking it as "Confirmed" be presumptuous?)