There are several reasons for bubbles in the sight glass. If one of the traditional refrigerants showed vapor in the sight glass it often meant there wasn't enough liquid refrigerant being fed to the valve, and more refrigerant was added to the system.
Blends could show flashing for the same reason, however, they can also flash when there is plenty of liquid in the receiver. Ironically, this liquid in the receiver could be causing the problem, particularly when the equipment is in a hot environment. Blends will come out of the condenser slightly subcooled — at a temperature below the saturated temperature of the blend at the existing high side pressure.
Yet when the blend sits in the receiver, it can "locally fractionate," or change composition slightly by shifting one of the components into the vapor space of the receiver. This will effectively produce a saturated liquid in the receiver, at the same pressure you had before, which flashes when it hits the expanded volume of the sight glass. In most cases these bubbles will collapse when the blend gets back into the tubing which feeds the valve, and the system will operate just fine.
Check other system parameters such as pressures, superheat and amperage to confirm whether you have the right charge. Don't rely solely on the sight glass.