The BBC reports that later today, Foreign Secretary Dominic Raab will face Sir Keir Starmer in the first Prime Minister's Questions since the latter became the new Labour party leader. The Foreign Secretary is deputising for the Prime Minister as he is still recovering from COVID-19.
Usually, when either the PM or the Leader of the Opposition can't attend PMQs, their counterpart will also appoint a deputy to field or ask questions respectively. See, for example, this article from parliament.uk in 2014 which states:
When the Deputy Prime Minister takes Prime Minister's Questions, the Opposition questions are usually led by the Deputy Leader of the Labour Party, Harriet Harman.
Or, more recently, when Raab faced then Shadow Home Secretary Diane Abbot:
In the battle of the deputies, it's hard to triumph - and disaster is never far away. But neither of those imposters muscled into Diane Abbott and Dominic Raab's first Parliamentary duel.
When was the last time this convention was broken, as it appears will happen today?