What does it mean when a battle resolves with no casualties on either side?

[Mindel]

I have a situation where my army (some 40k) was attacked by a larger force (about 60k). After a hard fight, my army loses and retreats back one province. The attacking army follows it and attacks it again.

This time, the battle resolves with 0 losses on either side, with my army losing again and retreating one more province back.

The attacking army follows it yet again and this time finally stackwipes my army.

What was that second battle with 0 casualties supposed to be about? Is this a bug?

 

[bokorthedust]

I might be wrong, but I think I've seen it happen when one of the sides has absolutely no morale. They fight for the mandatory 12 days (it's 12 days, right?) then retreat again, but get a morale tick at the end of the month, then the third battle happens and they get stackwiped because they are small and run out of morale before they could retreat. I'd definitely say it's a bug of somekind.