Slitherine

this may seem silly, but it bugs me that I can't get the 4th star in Blitzkrieg France, for 'Complete the Mission in 15 turns'.

A. I capture both Bridge objectives in 15 turns, or 13 or 12 in easy mode.
B. I destroy not only 30 units but 50 units, enough to get the achievement, in 15 turns, or 14 or 13 in easy mode.
c. I kill 3 Francs Tirreurs units, enough to get the achievement, usually in 6 or 7 turns.

so, why don't I get the complete mission achievement star for completing the mission in less than 15 turns? What is left in the mission?

It looks like you actually need to destroy all enemy units in 15 turns to get this achievement.