Every Torah reading is followed by half Kaddish, except for Shabbat and fast day mincha.
Mishnah Berurah #4 on O.C. 292:1 says that on Shabbat and fast days we don't say Kaddish after Torah reading because there is really nothing interrupting the Torah reading and the Kaddish that's said just before Amidah, so that Kaddish works for the one that should be said after Torah reading.
This reasoning seems to work for Shabbat. But, on fast days a haftarah is said. WHy does Mishnah Berurah not consider that as a hefsek (interruption)?