Many times now I have noticed a lot of my photos are coming out out of focus and I know that I have focused on the eyes of the subject in each photos. I notice it more with any focal length below 100mm but it's not always happening. It seems that if the shutter speed is slower it seems to be much blurrier. I've attached photos to show what I keep getting with my shots. I use a Canon 5D MK IV. Would you say that the lens is back-focusing?
First photo is 1/500 sec at 4/3.5 ISO 320 70mm
Second photo is 1/250 sec at f/3.2 ISO 320 70mm
Third photo is 1/640 sec at f/2.8 ISO 320 70mm
I can't see anything sharp in your second image, either behind or in the small amount of the image to the front of the subjects.
The best way to tell if the focusing is off is set the drive and focus for single shot, choose decent lighting so you can keep the shutter speed at 1/1000, set the AF to use a single microfocus point in the middle of the screen, and set up a target to photograph so that you can see where the lens is actually focusing.
You can use as simple as a stick stuck in the grass, put the focus point on the stick, and closely examine the grass (transfer images to your computer to see them in large format) to see whether the grass is most cleanly in focus in the plane of the target, in front of, or behind the target. When needed, it is easy to adjust the focus calibration with the 5D4 and other late model Canon DSLR bodies.
The old rule about shutter speed at least equal to the reciprocal of focal length is just a rough guide. If you have the skills and form of a sharpshooter, then you can handhold at much slower shutter speed. If held casually, motion blur due to camera shake can easily occur even at pretty fast shutter speed.
The very first thing to do is reset the 5D Mk IV, fantastic camera BTW, to default settings. Menu>Tools>Clear all settings>Clear all custom settings. Very important you make sure you do the clear all custom settings. Make sure any micro focus adjustment is set to zero. Remember micro focus does not make a lens any sharper. It simply moves the critical focus point either forward or backward. All lenses are as sharp as they'll ever be on day one of creation. This is not to say that something can't go awry inside later on, though.
Now you can try Rodger's trick for AF. It would be great if you have more than one lens to do at the same time, too. The reciprocal of the lens FL is just a guide. And, as we age that rule may need to be upped a bit. Say even doubled!