Make an anim (lets call it Idle), ideally something with very low movement (as this makes the issue easier to see).
Set the Idle to additive anim type (local)
Change the base pose type to selected animation frame
Select the Idle as the ‘selected animation frame’
This should make an additive which is zeroed as it is using itself for the start frame
Play back the anim (carefully scrub through if needed)
Around frame 2/3 you will see the animation drift from where it should be causing a small pop
I only noticed this because the animation in question was full screen and very subtle.
This only happens when you use a selected animation frame which matches the first frame
I tested this by creating a frame which is slightly different to the first frame of the Idle and then using this for the selected animation frame. This worked fine and I was able to make this (let’s call is Idle Offset) an additive itself and add it together with the Idle additive to get the result I wanted.