sometimes the elbow gets hotter than the rest of the kettle causing the wort in the elbow to boil, I don't think it's a huge deal but if your spouting out the top a simple piece of metal flashing on your burning and under the elbow should resolve your issue.
happens to me all the time. no worries. gauge volume at the beginning and at the end. overkill banjo burners tend to heat everything up beyond where it needs to be. If your sight glass is open at the top, add a piece of metal as a heat shield under it to keep it from boiling over.