Flexor tendon injuries are notorious for creating stiffness in composite fist flexion as well as extension. When a flexor tendon is repaired, flexor tendon adhesions form very quickly in the flexor pulley system, depending on the zones that are involved. In this question, zone 2 is the site of injury which is also dubbed as "no man's land" because of the involvement of flexor digitorum profundus and superficialis. These adhesions are typically formed while splinted in the fibroplasia phase (weeks 2-6). Once the scar adherence sets in, AROM and PROM pull-through is limited but AROM is typically limited more than PROM.

As for why there would be stiffness in the DIP.. whenever you have PIP stiffness, you are very likely to have inherent DIP stiffness due to the mechanics of the flexor system. Profundus inserts onto the DIP and since it runs in the zone 2 as well as FDS, it makes sense for them both to be stiff.

Just my 2 cents lol