It’s almost certainly a fault. This is an L series lens too.
I’ve borrowed this exact lens and found it to be the best I’ve ever used. Almost immediate focus speed, steady as a rock whilst on IS. Sharp as a carpet tack where I needed it. Personally I don’t care about the outer edges as (a) there’s not usually much photo interest out there (b) I crop. However, I didn’t notice too many faults at the outer part of the photo – be it at the 100 or 400 end.