Is it normal for acrylic bathtub to be blue after drainage?

Well water comes out from the same position, and the pressure is high when the nozzle is down.

This is the basic knowledge of fluid mechanics.

Siphon principle is the principle of using pressure difference. In a closed container, the liquid has the same height and pressure. The siphon is full of water, but there is no air. When the water level at the inlet end is high, close the outlet with the palm of your hand or other objects. At this point, the pressure in all parts of the tube is equal. When all the dust settles, open the exit. Although the atmospheric pressure on both sides is equal, the water level at the inlet is high and the pressure is high, which pushes the incoming water to flow out from the outlet.