Perfectly normal. Mine bubbles a lot too. You will always get air in the return line as long as your return line isn't 100% filled at 100% capacity eith water. My sump has a simple baffle in it to get rid of the bubbles before the water passes into the refugium but this seems nice but unnecessary to be quite honest.

You could try using a micron bag on the end of the return line to get rid of the air bubbles and also trap floating debris.