Yes, this is a bug. A change from 4.0 to 4.1 reduced noise using some tricks and removed a special temporal AA pass specific to reflections, using just the final temporal AA to clean up the noise. When the final temporal AA isn’t used and FXAA is used instead the noise from SSR will never be cleaned up. I will fix this.
In the meantime if you wish to use SSR with FXAA set r.SSR.Temporal 1. This reenables the pass I disabled for 4.1. In either the default case of using temporal AA or using FXAA with r.SSR.Temporal the noise should be better than 4.0.
An additional bug with SSR+FXAA is the reflections won’t include translucency or fog.
I want to continue to improve the noise of SSR in the future but its a tug of war between reflection accuracy, performance, and noise.