I tried the EF50 f/1.8 STM on my R5 last week and it's by far the worst result among all my EF lenses. It gives a focus confirmation, but it is out of focus 90% of the time with moving subjects.
I have the same issue with my R.
I wanted something compact with a 50 and not a 35, but it is not so compact with the ring adapter, and quality is bad. Let's see what this RF50 can do