This came from the Fred Miranda Site, I have tried this and it helps
https://www.fredmiranda.com/forum/topic/1714543
R5 double back button focus - bug in firmware
I'm new here, so perhaps I missed it, but searching the forum I have found no discussion of this issue, and it seems pretty important.
Double back button focus is a superb new possibility with the R5. The common way I have seen to implement it is to have the AF-ON button set to default as your left-BBF, initiating whatever focus method you have selected in the main camera settings, usually Spot AF. The * button is used as right-BBF to initiate Face-Tracking AF, and the obvious and widely advocated approach is to use the Customize Buttons menu to assign "Eye Detection AF" to the * button. But there's a problem with this.
BACKGROUND
The issue relates to the setting in menu AF5.1 "Initial Servo AF point for Face-Tracking AF". If you're not familiar with this setting, it's best to play around with the main focus method set to Face-Tracking AF, and come back to double-BBF later. The default setting in menu AF5.1 is the third option AUTO. In AUTO, for Face-Tracking AF there is no small focus box displayed in the viewfinder, and the software just guesses what you want to focus on. Place two identical objects in frame (I use two bottles set on a table) and AUTO will initiate on the one closest to the center of frame. If you take this AF5.1 setting out of AUTO into one of the other two possible settings, in Face-Tracking AF you now see a small focus box that TELLS it where to initiate. You can move the small focus box anywhere in the frame with the joystick, and tracking will initiate on the object you specify. The difference between the two non-auto settings becomes apparent if you switch back and forth between Face-Tracking AF and Spot-AF. With menu AF5.1 set at the first option, the small box in Face-Tracking AF moves independently of the small box in Spot-AF. With menu AF5.1 set at the second option, they move together.
PROBLEM.
With dual back button focus I think most people don't want to be in the AUTO setting for menu AF5.1. When you push right-BBF to start tracking, you don't want the software to guess where you want to initiate tracking. You want tracking to initiate at the place where you were pointing the left-BBF Spot AF focus box. So you want menu AF5.1 to be set to the second option. The problem is that if you have set up right-BBF by assigning "Eye Detection AF" to the * button, there is a firmware bug that it just ignores your choice in menu AF5.1 and stays in the AUTO setting. You can prove this to yourself by putting two bottles on a table - go from left-BBF to right-BBF. When it's starts tracking it will always initiate at the object closest to the center of frame, even if you were pointing the Spot-AF focus box at something else.
SOLUTION
In Customize Buttons, don't assign the * button to "Eye Detection AF". Instead, assign it to "Register/recall shooting func". Notice that the INFO button then lets you register a setup. Un-check all except two things: AF method = Face-Tracking & AF operation = ON. The * button now operates in exactly the same way as your right-BBF to engage Face-Tracking AF, but now it DOES obey your selection in menu AF5.1.