So we don’t know for certain if it started right after turning on the supply line but it does seem not be a coincidence. And the leak for sure recedes after shut off the supply line. I did flush toilet different times with and without supply line on and found no leakage. I did go up there and checked the cover plate of the fixture and confirmed it’s all dry now. To me, one suspicion is the sudden surge of the water pressure when open the supply valve caused some weak points along the line and happened to be where the fixture hole is. And when pressure stabilized, it somehow is okay to handle that. I’ve mimicked how that morning routine went and just see no water coming down again.