PDAF systems work based on the amount of contrast that they can detect. Your test scene includes a poorly lit and fairly low contrast target in the same frame with a much brighter and much higher contrast area at a different distance from your target. Even if both areas were equally in focus the area to the left would appear, to our eyes, to be better focused. So your test, as constructed, is pretty much designed from the start to cause PDAF to fail.

Is it normal for phase-detect autofocus to be inaccurate...

Yes. Even the best systems aren't perfect. There's no such thing as a perfect AF system, and certainly not a perfect PDAF system.

Roger Cicala at lensrentals.com recently wrote a series about focus performance that is pretty detailed and touches on several of these issues. It is a lot of material to go through, but I found it interesting reading.

Autofocus Reality Part 1: Center-Point, Single-Shot Accuracy
Autofocus Reality Part 2: One vs. Two, Old vs. New
Autofocus Reality Part 3A: Canon Lenses
Autofocus Reality Part 3B: Canon Cameras
Autofocus Reality Part 4: Nikon Full Frame

What I would like to know is if the severity in my example shots is normal and why it only happens at 18mm.

The reason it is more pronounced at 18mm is simply because the area of sensitivity for each AF point on the PDAF array buried in the floor of your camera covers a greater area of coverage of your target as you increase the angle of view of your lens. And those areas of sensitivity are often much larger that those little squares in the viewfinder!

The PDAF system in your 750D/Rebel T6i is pretty much a clone of the PDAF system introduced in the EOS 7D back in 2009. A few features available in the original 7D are not available in the 750D. This is not due to a hardware difference but rather a limitation in the firmware that reflects a choice by Canon to only include certain features in their higher tier cameras. As such, this answer to another question contains information highly relevant to your question. Pay particular attention to Roger Cicala's AF Reality, part 3b blog entry. The 7D (and 70D, 750D) PDAF system, though very configurable, is not exactly Canon's most consistently accurate one.