In short NO
>The move Stored Power's base damage is increased by 20 times the total sum of the user's positive stat stages: that is, its initial base damage is 20, but if one of the user's stat stages is 3 and another is 1, its base damage will be 20 + 20*4 = 100. -Source
So it wont lose any BP when it goes Sigilyph gets a negative stat change, but it also wont get a increase in BP when it gets back to normal. It doesnt have any positive stat changes yet, because the positive change was canceled out by the negative. Stored Power will still have a BP of 20 until that Sigilyph has +1 Defense and not neutral.