The air bubble is generated from the liquid in the membrane. During the production, a small volume of air would dissolve into the liquid, and under some circumstances, such as low air pressure on high altitude area or high temperature or after repeated operation, the air would come out from the liquid and gathering to be air bubbles that we see. Storing in low altitude could help the bubbles dissolving again back to the liquid, however, some of the air might not be able to dissolve again. Therefore, this is a normal phenomenon and the air bubble doesn’t affect the housing operation

I've seen bubbles come and go, nothing to worry about.