Sounds like you have a pump issue for the low or no assist while turning left. This is assuming the rack they installed is a good part. However, it is very unlikely that you would get another that does the same as your old one. With that assumption being made, the rack could be failing but we need to find out what is causing it to fail. The pump would be the next likely cause. They need to check the operation of the pump including pressure output when turning left. If the pump is putting out proper pressure then something inside the rack is not allowing pressure to that side of the rack. I attached a couple documents about your specific vehicle that will help explain this.
Either way, if this was a reputable shop, they should inspect this and find out what happened. Technically, this is called a "comeback" which means they made a repair, it did not fix it, so the vehicle comes back.
The fact that this was gone and came back shows they at least fix the source of the issue but they missed the root cause of what was causing the rack to do this.
Images (Click to enlarge)
Wednesday, April 24th, 2019 AT 8:16 PM