All I will say is they must mostly only appear in the afternoon then because since they started in passenger service, I see a average of 32 services every weekday morning and not once have any of them been a short formed service.
Therefore I believe they're very much in the minority.
I get every shortformed train sent through to my work mobile so I can offer some specifics.
Today there are 14 Corby services running 4 vice 8 (2 unit diagrams, effectively) - 7 round trips.
Yesterday there were 28 Corby services running 4 vice 8 - 14 round trips.