(Two phase)Bubble shrink in a two phase flow

I want to simulate a bubble dynamics in the water.
I initialize a bubble in the water, and use phase-field model.
I give the water a velocity in a channel, the bubble moves with the water.
After a few time steps, the bubble become smaller and smaller, than disappear!!!
It is very odd, as I set incompressible flow and a uniform mesh.
Other parameters are default.
What is the reason for this phenomenon.

Thanks.