When my pond was filling after renovation it would drop like that after every good rain event.

Up some, down some, up some more, down some more. The more it went up at a shot, the more it dropped after that shot.

It took six months, maybe a little longer, for it to stabilize, and I was a nervous wreck over it the whole time.

You may have a small leak, but it's impossible to know that until the pond has spent some time at full pool and the soil surrounding the pond has become fully saturated.