Nifty fifty STM - focus issues, or human error?

[TMHKR]

I have a question for users experienced with fast primes. Today, I tried to test AF with my 700D (T5i) and my two lenses:

- EF-S 18-55mm 3.5-5.6 IS STM
- EF 50mm 1.8 STM

I noticed an issue with the 50mm. When I open up the aperture to the max, the AF jumps a bit on every other photo.

But here's the catch - when I put the camera to Live View, and focus using contrast-detect only, the focus is nailed every single time. But the moment I revert to phase-detect, the focus jumps every few photos. This was never an issue with my 18-55 lens.

After shooting wide-open, I matched the parameters of 50mm to 18-55 ones - that is, I selected 5.6 aperture (because 5.6 is max on 18-55 at the telephoto end). Phase-detect focusing improved significantly.

My question - is 50mm supposed to jump focus when wide-open? Why it doesn't jump when using contrast-detect?

Note that my camera doesn't have micro-AF adjustment, and doesn't have dual-pixel AF. It uses an old-school 9-point, all cross-type sensor (probably from 60D). I've noticed that phase-detect AF takes more time if set to apertures larger than 3.5.

Testing parameters:

- 700D and 50mm 1.8 STM, subject = a row of books
- global settings = 2 sec delay, wide-open, center AF point (dual cross-type), tripod
- first test with phase-detect, 4 shots, viewfinder = two shots good, two bad
- second test with contrast-detect, 4 shots, Live View, electronic 1st curtain = all good

When using phase-detect, it goes like this (note that camera and subject never moved):

- half press > focused > full press > countdown > shot
- half press > focus jumps > full press > countdown > shot

So is it okay, or should I replace my lens? Is it lens, or camera issue? Few days ago I took a photo of my brother, but it was out of focus (lens was wide-open). I tried to be as still as possible, yet the photo wasn't sharp.

But if we cannot use the widest aperture, what's the point of having it on the lens? Do I really have to resort to contrast-detect Live View to get sharp photos on 1.8? Or is it just how fast primes work? Yes, I know that DoF is very thin, but the camera was on tripod. Most people praise this lens, and I sincerely hope it won't be a big issue during real-life use.