This is what I get pretty much every time when shooting in Live View mode using the Canon 50 1.8:
This is what I get when I'm lucky when focusing through the viewfinder:
This is what I get when I'm not lucky (20-30% of the time):
What's moderately interesting (and kind of confirms this blog post from LensRentals) is that the kit 18-135 STM has a much higher success rate when focusing using the viewfinder.
Is there anything I can do to improve focus accuracy with the nifty fifty except shooting in Live View mode? I imagine other lenses like Tamron 17-50 2.8 will have similar issues?