Bubble = Trapped Air.
As you inject the plastic the air it is pushing has to go somewhere. If the lower vent is already covered by the tail color the air has no where to go.
So, it is best to cut the tail just below a vent. It is possible that some of the air might bubble up through the plastic to the next vent, but it is unlikely.
Give a look to where you are cutting the tail and see if that helps.