Actually Canon didn't flip the double gauss design, they used the configuration that they and others had used on rangefinder cameras. The 1964 Canonet rangefinder camera has a 45mm lens which is very similar to this RF one in its optical diagram. That design of double gauss was able to be placed closer to the film which suited mirrorless rangefinders of the day and is perhaps why Canon went down this route with modern mirrorless. So basically the RF 50/1.8 is a rangefinders version of the EF 50/1.8, so it was never going to be much different.
Please be aware, I said 'very similar' to the 1964 Canonet, not identical, lest forum member ace, that omniscient of lesser Canon lens design comes flying in on her broomstick again to bereate me for spreading false information !