Welcome back...haven't seen you post here in a long while....

Not sure if this is specifically related to your issue, but it should like it could be similar. Check out this blog post from Roger at lens rentals: https://wordpress.lensrentals.com/bl...-of-variation/

Specifically he talks about issues with the 24-70 2.8 (first version) about halfway through the article (there is a heading for "The 27-70 Mystery").

Stephen