Most likely the bubble (since you say it is 'distinct') is telling you that there is still too much hyperbolic-ness to the mirror and not enough spherical-ness. That is, the tool center is flat and the tool edge too curved. This is a result of long wide strokes used to hog out the curve and is a normal stage of mirror making. Stick with your 1/3 W strokes until the bubble goes away. Channeling the tool will help speed this along. Don't go past 220 grit until this is licked.