For a nickel defense, in formation view, MLB is on the left and SLB is on the right.
in custom playbook, MLB is on the right and SLB is on the left.
To be clear, I mean right and left from the perspective a defensive player facing the offense.
I noticed this in the 2020 version and it hasn't been fixed. At that time there were similar problems with other formations that I'm guessing haven't changed either.

Yup, thanks, updated the depth chart formation view to put the MLB in the middle. Will be available in a future patch.