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What body are you using it on?
Have you tried comparing CDAF to PDAF at a few distances (controlled) as a quick check to see if the lens is front/back focusing under certain conditions? Primes are a little easier to AF fine tune if necessary.
Heavy back lighting is rough on any AF system, as is extremely low light, since they need good light and good contrast for maximum accuracy. Also keep in mind it's a $200 lens, so while it obviously should work as advertised, I suspect the various tolerances are less than that of your 24-70/2.8G. Nailing AF @ F1.8 is not going to happen every single time, but it shouldn't be failing outright in poor lighting.
Everyone seems to have a copy of that lens, you could always try another copy, or head down to your camera store and try out their display lens in some poor lighting in the store. In lighting situations where I need F1.8 and ISO 1600-3200 for a reasonable shutter speed, my 50/1.8G works just fine on my D810. I don't think I've used it in any worse conditions than that yet.
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