R6 with 70-200 2.8 for marathon, not focusing accurate

[belgerphoto]

I have been a portrait photographer most of my 30 year career, but recently started sports photography and did a marathon yesterday. Used my R6 (M1) with EF 70-200mm f/2.8L IS II USM and adapter. I was in Tv mode, AI focus human eye detection with back-button focus set for continuous focus.  Eye detection picked up for the subject I wanted and I locked it in with back button (showing blue square on eye), but more than half the photos were focused slightly behind the subject with very few being tack sharp for subject selected.  Do I need to calibrate the lens? Is there a way to test the lens or camera for focus accuracy for moving subjects?  

My servo AF was set to Case 1

[deebatman316]

First STOP using AI Focus and use One Shot or Servo AF depending on the shooting conditions. There is no way to setup AFMA (Autofocus Micro Adjustment)/ AF Fine Tune on mirrorless camera. Since AF happens directly on the image sensor instead of separate sensor like a DSLR camera. Can you post some example pictures that are out of focus. A lot of older EF lenses aren't fully compatible with the highly advanced AF systems found in the EOS R series. A lot of them use older slower AF motors that simply don't have the speed to keep up with fast moving action. What drive mode are you using are you using H, H+ or a different drive mode.

[Waddizzle]

How many or which AF points are you using?

For moving subjects you should use Servo AF. Use One Shot AF for stationary subjects.  The AI Servo mode will yield unexpected results. Never use it. The only reason it’s in the camera is because the automatic shooting modes use it.  

When you use BBF with Servo AF you must maintain the button press to keep the AF system activated and tracking. 

I recommend disabling the AF eye tracking.  Allow the camera to focus on the closest object to the camera.  

[deebatman316]

@Waddizzle "AI" Servo was renamed to Servo AF in the EOS R series camera lineup in DSLRs it was called AI Servo. I don't know why Canon dropped the name "AI" from Servo AF. You're talking about AI Focus which definitely should be avoided. AI Focus and Servo AF are NOT the same thing.

[Waddizzle]

Thanks. Goes to show how often I look at that setting. Never. 

[belgerphoto]

Clarification to original post- I was using Servo, yes...called it Ai servo because was watching some Youtubes, and that's what they called it.

I went back and looked at the images in-camera so I could find the focal point, and see that many, if not all of the ones OOF are ones with no red square, which means I didn't hit the BBF, I just assumed since it was tracking that it was focused on that point.  Mid-way through the race I started pressing and holding BBF until the actuation, and those (ones with the red focal point square) are sharp. Ugh, learning experience on a tool I never use I guess.  

Folow up question- do you find any difference in the Servo AF Case selection?