Slitherine

I kept getting minor victories on Beiping, even though I had captured everything. On my fourth try, I realized what was happening. I was capturing too fast! The secondary requirement to keep all garrison units alive for 10 turns hadn't been met, because I had met the primary conditions by turn 9. So the only solution was to circle Langfang and wipe out its defenders, then sit around until after Turn 10 so all secondaries could be met before finally moving into Langfang.

You shouldn't get penalized with a minor victory for capturing everything too quickly.

Good point.
A solution could be to make the secondary objectives requiring a certain number of turns to happen, to be ticked from the start and only get unticked if the conditions are not met by that specific turn.