Whether fair or not, it breaches a standard Ximenean and Listener convention, but I've often observed in the past that Sabre is given more latitude by editors than is given to others.
Since 20A and 26A do not clash with any clued downs I assume the letter replacement must coincide with the unclued entry, which seems rather unfair, or perhaps it's in one of the unchecked cells.
I have in mind one possible entry for the unclued (8,4), which might be connected with something in ionacarr's last comment. Am I on the right track?