Guild war defense guardian kicks in too soon

Platform, device version and operating system:
PC
Screenshot or image:

What you were expecting to happen, and what actually happened:
In my 4th battle of last day’s guild war, the defender’s guardian sentinel kicked in way too sooner than I expected. It was supposed to kick in at turn 15, but in that battle it kicked in at turn 5 or 6. I didn’t count exactly, but at end of war I got +6 bonus from speed of battle, so it was turn 5 or 6.

How often does this happen? When did it begin happening?
I have vaguely noticed this bug in previous guild wars. But no concrete evidence until this time.

Steps to make it happen again
Don’t know. But I guess it may have some relation with extra turns.